Saturday, July 15, 2017

New CSpect ZX Spectrum Next emulator

UPDATE:  I've now added register setting, and memory window set/moving. You can also set breakpoints from the input line, and push/pop values.

I've been working away on a new debugger for my CSpect emulator, and it's now ready to use, so here it is along with my assembler (SNasm) and a sample showing how to use it all. Please read the readme and be aware it may go pop at any time. Aside from that -have fun!!

Download: CSpect Emulator

Thursday, July 13, 2017

Z8410 DMA chip for the ZX Spectrum Next

So the ZX Spectrum Next team have added a Z80 DMA chip, the Z8410  chip to be more precise. This was popular add on in Europe ( more info here: ), and has some very cool advantages for future games, especially because it's now a standard part. However information on it is a little thin on the ground, so after some major googling, I've found the info on them.

The DMA port can be 11 or 107, and it can transfer around 865k per second
From the web page above: Max. speed of data transfer on ZX128+ is...

17.3 kB(17727 bytes) / frame = 865.6 kB(886350 bytes) / second.

Or.... 1 byte every 4 T-States.

Now here is the really cool bit, because it's tied to the clock speed, when you speed up the NEXT (as it can go 7Mhz, 14Mhz and 28Mhz), the DMA will also speed up!  This is amazing, as it means you'll be able to transfer 1.73Mb/s, 3.46Mb/s, and an astounding 6.92Mb/s!  At 28Mhz you won't be using Layer 2, and that means you could copy the original Layer 1 screen in just 13.5 scanlines, which is incredible.

The registers are below, and here is the Datasheet

Saturday, July 01, 2017

esxDOS File access

So I was about to start adding some very basic "simulation" support for files into my emulator, and I'd lost track of the API details as they were listed on Facebook. Facebook isn't a great place to have technical discussions as you can't search later to find the stuff you need!

Fortunately I had it saved off, so before I (and everyone else) loses it, I thought I'd put it up here. I'll do a little ASM lib with all this later.
I'll also extend this if/when I discover more commands, as there appears to be very little info about the esxDOS API

; NOTE: File paths use the slash character (‘/’) as directory separator (UNIX style)

M_GETSETDRV  equ $89
F_OPEN       equ $9a
F_CLOSE      equ $9b
F_READ       equ $9d
F_WRITE      equ $9e
F_SEEK       equ $9f
F_GET_DIR    equ $a8
F_SET_DIR    equ $a9

FA_READ      equ $01
FA_APPEND    equ $06

; Function: Detect if unit is ready
; Out:      A = default drive (required for all file access)
;           Carry flag will be set if error.
             xor  a               ; A=0, get the default drive
             rst  $08
             db   M_GETSETDRV             
             ld   (DefaultDrive),a
DefaultDrive db   0

; Function:  Open file
; In:        IX = filename
;            B  = open mode
;            A  = Drive
; Out:       A  = file handle
;            On error: Carry set
;                      A = 5   File not found
;                      A = 7   Name error - not 8.3?
;                      A = 11  Drive not found
             ld   a, (DefaultDrive)  ; get drive we're on
             ld   b, FA_READ         ; b = open mode
             ld   ix,FileName        ; ix = Pointer to file name (ASCIIZ)
             rst  $08
             db   F_OPEN             ; open read mode
             ret                     ; Returns a file handler in 'A' register.

; Function:  Read bytes from a file
; In:        A  = file handle
;            ix = address to load into
;            bc = number of bytes to read
; Out:       Carry flag is set if read fails.
             ld   ix, 16384          ; ix = address where to store what is read
             ld   bc, 6912           ; bc = bytes to read
             ld   a, filehandle      ; a  = the file handler
             rst  $08
             db   F_READ             ; read file

; Function:  Write bytes to a file
; In:        A  = file handle
;            ix = address to save from 
;            bc = number of bytes to write
; Out:       Carry flag is set if write fails.
             ld   ix, 16384          ; ix = memory address to save from
             ld   bc, 6912           ; bc = bytes to write
             ld   a, handle          ; a  = file handler
             rst  $08
             db   F_WRITE            ; write file

; Function:  Write bytes to a file
; In:        A  = file handle
; Out:       Carry flag active if error when closing
             ld   a, handle           ; a  = file handler
             rst  $08
             db   F_CLOSE

; Function:  Seek into file
; In:        A    = file handle
;            L    = mode:  0 - from start of file
;                          1 - forward from current position
;                          2 - back from current position
;            BCDE = bytes to seek
; Out:       BCDE = Current file pointer. (*does not return this yet)
             ld   a,handle           ; file handle
             or   a                  ; is it zero?
             ret  z                  ; if so return
             ld   l,0
             ld   bc,0
             ld   de,0
             rst  $08           
             db   F_SEEK

; Function:  SetDirectory
; In:        A    = Drive
;            HL   = pointer to zero terminated path string ("path",0)
; Out:       carry set if error
             ld   a,(DefaultDrive)   ; drive to change directory on
             ld   hl,Path            ; point to "path",0 to set
             rst  $08           
             db   F_SET_DIR  

; Function:  Get Directory
; In:        A    = Drive
;            HL   = pointer to where to STORE zero terminated path string
; Out:       carry set if error
             ld   a,(DefaultDrive)   ; drive to get current directory from
             ld   hl,Path            ; location to store path string in
             rst  $08           
             db   F_GET_DIR  

Sunday, June 04, 2017

ZX Spectrum Next - Bitmap example disassembly

I've been having a poke around in the ZX Spectrum Next Bitmap example. It's mostly clear, but one or two things are....odd.
This is what I have so far.

UPDATE: Banking registers now confirmed by Jim Bagley

; Disassembly of the file "bitmaps\BMPLOAD"
; BMP Port $123b
;     bit 0    -    Write enable. (banks in 16K from $0000-$3fff)
;     bit 1    -    bitmap ON
;     bit 4    -    Layer 2 below spectrum screen
;     bit 6-7  -    Which 16K bank to page into $0000-$3ffff
; On entry, HL = arguments

; Since dot commands run at $2000, and the Layer 2 is paged into $0000-$3fff
; It needs to have some code above the lower 16K to actually copy into layer 2
; So copy up the "copy" routine, and it'll page in/out the layer 2 bank
2000 226221    ld      (HL_Save),hl     ; Store HL
2003 216421    ld      hl,UploadCode    ; Src  = 
2006 110060    ld      de,6000h         ; dest = $6000
2009 011f00    ld      bc,001fh         ; size = 31 bytes
200c edb0      ldir    

200e 2a6221    ld      hl,(HL_Save)     ; get HL back
2011 7c        ld      a,h
2012 b5        or      l                ; is hl 0?
2013 2008      jr      nz,201dh         ; if we have an argument, carry on

2015 213121    ld      hl,2131h         ; get message
2018 cd1d21    call    PrintText        ; print the filename
201b 1823      jr      Exit             ; exit

; Load file....?
201d 112421    ld      de,FileName      ; "filename.ext" text - space to store filename?
2020 060c      ld      b,0ch            ; b = $0c (max length of allowed filename - no path it seems)

; Copy, and validate characters in filename
2022 7e        ld      a,(hl)           ; first byte of filename
2023 fe3a      cp      3ah              ; is it a ":"?  End of filename
2025 2810      jr      z,EndFilename    ; if so end of filename  
2027 b7        or      a                ; 0?
2028 280d      jr      z,EndFilename    ; if so end of filename
202a fe0d      cp      0dh              ; newline?
202c 2809      jr      z,EndFilename    ; if so end of filename
202e cb7f      bit     7,a              ; over 127?
2030 2005      jr      nz,EndFilename   ; if so end of filename
2032 12        ld      (de),a           ; copy over to filename cache
2033 23        inc     hl               ; next src letter
2034 13        inc     de               ; next dest letter
2035 10eb      djnz    2022h            ; copy all
2037 af        xor     a                ; Mark end of filename
2038 12        ld      (de),a           ; store filename
2039 dd212421  ld      ix,FileName      ; get filename base address
203d cd6220    call    LoadFile:

2040 af        xor     a                ; no error
2041 c9        ret                      ; exit

; esxDOS detect if unit is ready
2042 af        xor     a                ; Detect if unit is ready
2043 cf        rst     08h              ; call esxDOS
2044 89        db      $89              ; M_GETSETDRV

; Open
; IX= Filename (ASCIIZ)
; B = FA_READ       ($01)
;     FA_APPEND     ($06)
;     FA_OVERWRITE  ($0C)
2045 dde5      push    ix               ; ix = filename
2047 e1        pop     hl               ; If a DOT command (rather than normal memory), uses HL not IX
2048 0601      ld      b,01h            ; b = FA_READ
204a 3e2a      ld      a,2ah            ; a = unknown  (a=0 for open)
204c cf        rst     08h              ; call esxDOS
204d 9a        db      $9a              ; F_Open
204e 325620    ld      (2056h),a        ; Store file handle  (self modify read from file code)
2051 c9        ret                      ;

; esxDOS command - Read from file - command $9d
; IX = address to load into
; BC = number of bytes to load
; A  = file handle
2052 dde5      push    ix               ; IX = where to to store data
2054 e1        pop     hl               ; get address to load into
2055 3e00      ld      a,00h            ; $00 is self modified
2057 cf        rst     08h              ; call esxDOS
2058 9d        db      $9d              ; F_Read
2059 c9        ret     

; esxDOS command - Close File - command $9b
; A = file handle
205a 3a5620    ld      a,(2056h)         ; Get open file handle 
205d b7        or      a                 ; is it 0? (did it open)
205e c8        ret     z                 ; if file handle is 0, return
205f cf        rst     08h               ; Call esxDOS
2060 9b        db      $9b               ; F_Close
2061 c9        ret     

2062 dde5      push    ix                ; remember filename
2064 cd4220    call    DetectUnit        ; detect unit and open...?!?!?
2067 dde1      pop     ix                ; get filename back
2069 cd4520    call    OpenFile          ; OpenFile - again??
206c dd21e720  ld      ix,BMPHeader      ; Read the BMP file header
2070 013600    ld      bc,0036h          ; read header ($36 bytes)
2073 cd5220    call    ReadBytes
2076 dd218321  ld      ix,BitMap         ; read block into $2183
207a 010004    ld      bc,0400h          ; read palette - 1024 bytes
207d cd5220    call    ReadBytes

; Convert the 24bit palette into a simple RRRGGGBB format
2080 218321    ld      hl,BitMap         ; Get buffer address ($2183)
2083 11003f    ld      de,3f00h          ; Dest address of converted palette
2086 0600      ld      b,00h

2088 7e        ld      a,(hl)            ; get BLUE byte
2089 23        inc     hl                ; move on to green
208a c620      add     a,20h             ; brighten up a bit? (blue is always pretty dark??)
208c 3002      jr      nc,SkipSatB       ; overflow? 
208e 3eff      ld      a,0ffh            ; if overflow, then saturate to $FF

2090 1f        rra                       ; get top 2 bits only  RRRGGGBB
2091 1f        rra     
2092 1f        rra     
2093 1f        rra     
2094 1f        rra     
2095 1f        rra     
2096 e603      and     03h               ; and store them at the bottom
2098 4f        ld      c,a               ; c holds current byte

2099 7e        ld      a,(hl)            ; get GREEN
209a 23        inc     hl                ; move onto red
209b c610      add     a,10h             ; brighten up a bit as well
209d 3002      jr      nc,SkipSatG       ; if no overflow, skip saturate
209f 3eff      ld      a,0ffh

20a1 1f        rra                       ; get 3 bits of green into right place
20a2 1f        rra     
20a3 1f        rra     
20a4 e61c      and     1ch               ; mask off remaining lower bits
20a6 b1        or      c                 ; merge with output byte
20a7 4f        ld      c,a               ; store into output

20a8 7e        ld      a,(hl)            ; get RED
20a9 23        inc     hl                ; move to next byte of colour (assuming alpha)
20aa c610      add     a,10h             ; brighten up a bit
20ac 3002      jr      nc,SkipSatR       ; no overflow?
20ae 3eff      ld      a,0ffh            ; Saturate to $FF

20b0 e6e0      and     0e0h              ; keep top 3 bits
20b2 b1        or      c                 ; merge with output pixel
20b3 12        ld      (de),a            ; store converted pixel
20b4 13        inc     de                ; move to next ouput pixel
20b5 23        inc     hl                ; move to next BGRA pixel
20b6 10d0      djnz    ConvertionLoop    ; c = pixel....?!?!?!

20b8 06c0      ld      b,0c0h            ; bc=$c000

20ba c5        push    bc
20bb dd21003e  ld      ix,3e00h          ; $3e00 = Destination address
20bf 010001    ld      bc,0100h          ; read 256 bytes of data
20c2 cd5220    call    ReadBytes         ; Read from file

; convert 256 value palette index into actual RGB byte pixel using palette lookup
20c5 2e00      ld      l,00h             ; l = xx (loop counter)

20c7 263e      ld      h,3eh             ; hl = $3exx
20c9 5e        ld      e,(hl)            ; e = Get palette index
20ca 163f      ld      d,3fh             ; d = $3f palette base address - 256 byte aligned
20cc 1a        ld      a,(de)            ; a = palette value (24bit converted downto 8bit)
20cd 265b      ld      h,5bh             ; hl = $5bxx converted 256 byte buffer
20cf 77        ld      (hl),a            ; ($5bxx) = converted colour pixel
20d0 263e      ld      h,3eh             ; hl = $3e00
20d2 2c        inc     l                 ; do 256 bytes of this....
20d3 20f2      jr      nz,CopyLoop

20d5 c1        pop     bc                ; bc = $c000
20d6 c5        push    bc
20d7 cd0060    call    6000h             ; block transfer 256 bytes of the bitmap
20da c1        pop     bc
20db 10dd      djnz    ConvertUploadLoop
20dd 013b12    ld      bc,123bh          ; Bitmap port
20e0 3e02      ld      a,02h             ; 2 = enable and visible
20e2 ed79      out     (c),a             ; switch bitmap layer 2 on
20e4 c35a20    jp      205ah

BMPHeader:     ds   54                   ; $20e7 to $211c

211d 7e        ld      a,(hl)            ; get character
211e 23        inc     hl                ; move to next one
211f b7        or      a                 ; is this 0? 
2120 c8        ret     z                 ; if so... end of string
2121 d7        rst     10h               ; outchr()
2122 18f9      jr      211dh             ; print all characters

; Looks like data of some kinds
FileName: db  "filename.ext",0
Message:  db  ".picload  to load image to background",$d,$00
HL_Save   dw  0

; Copied up to $6000
2164 05        dec     b                ; b = upper byte of memory address
2165 78        ld      a,b              ; get 256 byte page into a
2166 e63f      and     3fh              ; ignore top bits
2168 57        ld      d,a              ; de=$0000-$3ffff 
2169 1e00      ld      e,00h            ; Page bitmap block into lower 16K
216b 78        ld      a,b
216c e6c0      and     0c0h             ; get the 16K bank to page
216e f601      or      01h              ; OR in bank active bit
2170 013b12    ld      bc,123bh         ; bitmap register
2173 ed79      out     (c),a            ; map bank into memory perhaps?
2175 21005b    ld      hl,5b00h         ; $5b00 src image chunk
2178 010001    ld      bc,0100h         ; 256 byte copy (one line)
217b edb0      ldir                     ; copy up  (de=dest)
217d 013b12    ld      bc,123bh         ; bitmap register
2180 ed69      out     (c),l            ; l=0, disable current bank and screen
2182 c9        ret     

2183 00        nop                      ; file is loaded into here....

Friday, April 28, 2017

Hacking ZX Spectrum Manic Miner

So I was playing around with the original Manic Miner game, using a ZX Spectrum .SNA snap shot file (so an actual memory image), and while extracting levels etc. I also extracted graphics - which were obviously included as well.

For the record, I didn't reverse engineer all this, I just used one of the many pages out there describing it.  ( )

But it also occurred to me that it might be nice to have these graphics readily available for folk to play with, it would certainly have been handy when I was doing my remake a few months ago! So here's what I've extracted....

So, this is the baddies that come with each level, the items (1 per level), the 8 tiles (left to right per level) in colour, and in B/W, and then Miner Willy himself. There are several "special" baddies, and if I extract them I'll add them here as well.

I might see if I can go and grab the Z Spectrum font from the ROM, because I couldn't find it as a simple strip at the original size.

I'll just mention the sprites move through the graphic, so the animation frame and X position is usually....

frame = 3-((x>>1)&3);
draw_xpos = (*x&0xfffffff8)


ZX Spectrum 8x8 pixel Font

Sunday, September 25, 2016

ZX Spectrum colour clash on modern hardware

There are always some games that like to do very retro looking games, usually in the style of a Spectrum since you still get "hi-res" from it, rather than the C64 or something where pixels were doubled and couple look a bit nasty, even for today's retro fan club. However, as retro as they look, they never really capture the full spirit of the old machine because of one thing - colour clash.

The ZX Spectrum's screen consisted of a hires (for its day) bitmap of 256x192 pixels, and a colour (or attribute) map. The pixels in the bitmap would get coloured every 8x8 pixels depending on the colour in the attribute map.

So, roll onto modern day. We no longer have separate pixels and attributes so simulating this old mode can be a little tricky.

Roll on further and I suddenly realised a really simple way of achieving this using the hardware we all have in our machines already. So lets say we simulate the way the spectrum drew things, a bitmap screen an a colour map. First, lets draw everything in "white" pixels only - which is much like the spectrum would be, it would look something like this.

If we then drew the colour map, we'd just end up with big blocks of colour going over the top of the screen - like this

And this is where modern hardware struggles. However... if we could somehow use the first image, as a "mask" for the second, then we could get rid of the pixels from the blocky colour image and get the image we wanted. We could generate both these images and then send them through a shader and do - well, whatever we want to, but there's actually a simpler way.

Let me introduce you to destination alpha. DestAlpha is the alpha channel on the screen surface (or render target) that you render to all the time anyway. Whenever you draw a sprite the RGB goes onto the surface from the texture/sprite your drawing, and so does the alpha. When you clear the screen to black with no alpha (or UINT32 of 0x00000000), and then draw a sprite onto it, you're effectively making a copy of a sprite. When you start drawing lots of other sprites it all gets a bit of a mess and is basically useless, but if you control can be very handy

There's also another string for our bow; the colour channel write mask. This lets you switch off and on the different channels of the screen (red,green,blue or alpha). Now it gets very interesting.....

Lets say that black and white image above was also represented in via the alpha channel.
The image shown here has white pixels where there is data, and transparent everywhere else. This is a "mask" of the data we want to draw, and if we put the same data into the alpha channel we can use "destalpha" to actually mask everything.

So, the first thing we need to do is draw the background - whatever we want, however we want. Next we need to clear the alpha channel only - leaving the colour screen intact. We do this by setting the colour channel mask so that only the alpha channel is being written to. We can then render a filled rectangle with alpha of 0. One the alpha channel is cleared, we then set the colour mask back to normal and render everything else.

Once this is done,  the alpha channel will look something like the transparent image above, a lovely alpha mask - now if only we could add this to a sprite it would be like any normal texture....  Step in dest alpha. Normally were use source alpha, inverse source alpha as the blend mode. This would normally take the alpha value from the source i.e. the texture, but now we'll use destalpha, inverse destalpha. This will use the screens alpha channel as the alpha for the blend.

Now when we render the solid rectangles of colour that would be the colour mask, the underlying pixels that crated the alpha channel, will only let colour through were we drew a pixel in the first place.

So, by using the transparent image as the alpha channel of the blocky image, we're left with the image above - which is just like the spectrum.

Now comes the good bit. When we render objects - players, baddies, pickups etc, all we need to then do is change the colour block in the blocky image (which is rendered layer), and then this gives us the same blocky, colour clash feel that the spectrum had

Here's the general order to rendering things....

Render "paper" colour map  (32x24 of 8x8 blocks of colour)

Set colour mask to alpha only

Disable blending and alpha-test
Draw giant flat rectangle of 0 alpha, and 0 colour
Reset colour mask to full ARGB
Enable alpha blend and alpha test

Draw everything

Set colour write mask to RGB - disabling ALPHA

Enable alpha blend and alpha test
Set blend mode to DestAlpha, Inv_DestAlpha

Draw all colour blocks (32x24 of 8x8 blocks of solid colour)

Set blend mode back to SrcAlpha, Inv_SrcAlpha

Once you've done all this.... you'll be using the "pixels" from your sprites, and the colour from the attribute map. Whats better is every bit of hardware out there can do this. It's all standard blend modes stuff - even older hardware. If you've done all this, you should get something like the video below. Make sure ALL your sprites background graphics are transparent, and don't have solid pixel colours (like black) as this will effect the mask.

One final point... It should be noted that the "colour" pixels written at the same time of the mask aren't really needed, but if you colour them as they are meant to be (not just white), then it means you can easily switch the effect on and off without affecting the game. This gives the end user the option to have colour clash or not, which is a nice side effect.

Friday, January 01, 2016

Why create a GameMaker export to the Raspberry Pi?

Okay, these are very much personal opinions...
So I've been asked a couple of times why bother? It's a tiny machine with (probably) zero chance in making money from it - so why?

First, the Raspberry Pi is an amazing thing, Eben Upton and his various partners have done a phenomenal job getting it out in the first place, never mind the subsequent work of getting the Pi Zero - which is beyond amazing!

What I love about the Pi more than anything else, is that you no longer have to decide to take out a second mortgage on your home to give your kid a computer in their room! What's more, you no longer have to worry about them breaking it. At $25 you can just get another one. With the Pi Zero, this is and even simpler decision. Having a computer in my bedroom when I was growing up was life changing. Without that, I'd never be where I am now.

Of course.... there were other factors that helped. First - content. Without games to play, I'd have grown bored very quickly, and my ZX81 would have ended up in a drawer. What you need to inspire development is inspirational content. You want to play games, use programs and hardware and think - "That is SO cool! I want to do that!"  This is what drove me. We used to play the simple games available, type in programs from magazines, and spend hours trying to do stuff ourselves. 

Put simply, this is what I want to help achieve. With GameMaker, we have access to users who can provide these inspirational games, games that you want to play over and over, games that make you want to try and create stuff yourself.  Sure, there are limits. This isn't a Quad core i7 with 16 Gigs of ram and enough HD space to store most of the internet on. don't have to be full 3D ultra-realistic to be good. Vlambeer have some amazing games, Locomalito has made some astounding games - ALL FREE!  All they need (in theory) is a port of the GameMaker runner to help make it happen.

So... that's my reason number one. It's how I started, and if I can help others do the same, I'm all for that.

Second - Education. Not so much for game creation itself, but for Electronics. Currently most folk seem to use Python to do this, and to be frank - python sucks. Even with simple GPIO commands I've been having great fun with it. Remote development is ideal for this kind of thing, because when things go wrong and the Pi crashes (as it inevitably will), you can simply reset while your entire dev environment is safe and sound on a separate machine.  At some point I'll get the remote debugger working as well (It almost works now), and that means you'll be able to step through your code and development will be even easier.

On top of this there is an emerging home brew arcade scene. Folk like Locomalito take their games and stick them in PCs inside arcade machines - like a mame cabinet does, and then run their games like in the old days, complete with arcade buttons and joysticks. These days, they appear to be moving more towards using the Raspberry Pi - it's cheap, has lots of easy to use pins for interfacing and if it blows up - meh, they can buy another one. The runner port will allow games to be run on the machine, while the GPIO commands I've added will help them interface with buttons and joysticks easily.

Lastly... a word about the Pi Zero. I can't begin to say how much I'm impressed they made this happen. It's such an incredible break through, They even gave one away on the cover of a magazine! I'll let that sink in...

They gave away a whole computer on the cover of a magazine.

Wow.....It has the same GPIO pins as the PI, and while only a single core, it is a 1Ghz core. This means you could make a case for it with a 3D printer, get some very cheap SD cards, and then sell your game as a plugin console - probably for $20 or so. That astounds me. Even without the case etc. you could sell you game - complete with computer, for less than $10.  You can buy 50 micro 4Gb micro SD cards for £1.36 each, add that to the price of the Pi ($5) and you're still going to make a profit at $10. Wow.

Before the Pi came out, we tried to get GameMaker 7 actually running ON the machine. We almost got there, but ran out of time. We were doing it for free because we believed in what they were doing, but commercial pressures reasserted and we had to move on. Long term, I'd love to see a GameMaker IDE of some kind running ON the Raspberry Pi. It would be an amazing thing to behold, and would make learning so much more fun. day day.

Thursday, December 31, 2015

Hooking up an SD card to the RaspberryPi

So I've not done a personal blog post for fricken ages! Mainly because any post I would have done would have been a GameMaker one, so they've been on the YoYo Games Techblog instead.

While porting the GameMaker: Studio runner to the Raspberry Pi, I thought it would be very cool to allow access the GPIO (general purpose input/output) pins. This would let you use GameMaker to do some cool electrical experiments without A) having to resort to using Python (yuck) and B) allow remote development - meaning you don't have to work on the actual Raspberry Pi itself - and possibly even remote debugging using the GameMaker debugger. All of which would be pretty cool.

I've already hooked up 2 different LCD screens, so I wanted to do something actually useful. Having a secondary SD card that isn't controlled by the OS could be pretty useful - especially if you want to somehow make a console or something. You could boot up into Linux on the OS card, but then allow games to be plugged via this dedicated card - well that's the theory.

I've done SD card coms before with a Commodore 64, but it was some time ago, so I was almost starting fresh. Starting any new project is always annoying, you're never sure if it's a software bug, or an electrical one. It's doubly tricky for a noob like me, as I've only just toyed with electronics, it's mostly guess work. Still, that's never stopped me before! I had a couple of false starts, but finally got the connections right, the final layout is shown here...

Because the Raspberry Pi's GPIO is 3.3v, I was able to just connect the pins directly, if they were different (5v for example) then I'd have had to alter the pins voltage before the hit the card. But you don't need that with the Pi which makes life really simple - although I have been reminded that I should really use a capacitor to smooth things out, but this does work for just hacking around.

Once connected the real fun of trying to talk to the damn thing starts! SD cards have a very cool 1 bit SPI interface, where basically, you set bits and toggle a line up and down and the card accepts that bit. Send lots of these, it gets a byte - and so on. It also sends data in the same way, just toggle lines up and down and accept bits at your leisure!

So first, lets initialise the ports/data lines, and then we can get to throwing some data at it. Below is the code (written in GameMaker: Studio obviously) that sets up the ports and line direction. It's worth mentioning that the Pi can set lines to input or output, so you can see below that I setup most to be output, and the DataIn line to be input.

The CS is the chip select line. Whenever you want the card to accept your data, you set this line low (to 0).
The CLK line is the line we toggle Low/Hi to acknowledge that a bit has been sent or received.
DataIn/DataOut are the lines for the actual bit data we're reading/writing.

All in all, it's pretty simple. The Pi does have hardware SPI lines, but I'm not using these yet as I've yet to add support to GameMaker for them.

Once we've set these up, we really need to start trying to initialise the SD card - or MultiMediaCard interface as that's what used it first. Before getting going, we really need a bit and byte sending routine. This is the lowest level of function we'll use, and it'll handle all the bit flipping we require.

The function to send a bit is very simple.  We set the bit to 0 or 1, set the CLK line LOW(0) wait for a bit - this depends on the speed of your CPU, but on the PI,it's a small (or non-existent) delay. We then read a bit in. We do this because it's easier to have a single function that reads and writes, rather than having to worry about the state of the DataOut line while trying to use the DataIn line. This just makes life simpler. We then take the CLK line HIGH again (1), and pause again. After that...we're done, just return the bit we received.

/// SendBit(bit)
var b = gpio_get(DataIn);
return b;

With this setup, we need to be able to send a command, and this the sequance you need(image from )

Command/Response sequence

You can see from the image above, we basically send bytes of Info to the card using the SendBit() function above for each BIT in the byte. The send command function is pretty simple once you know these two facts.

Once you have the Write8() function (which is just a loop of 8 around the SendBit() function) and the SendCMD() function, we really just need to actually use them to initialise the SD card. The core of the initialisation centres around sending the CMD0 and CMD1, but before doing this you have to pulse the SPI line a lot to set it into SPI more. This means sending "more than" 74 clock pulses while the CS and DI lines are HIGH. All commands should return a 0 if they are accepted, or an error code if not.

The InitCMDS() just sets up the command values, CMD0=$40, CMD1=$41 and so on, while InitFAT() is setting up stuff for reading the actual disk later...

After we've done this, then the SD card is ready to actually send us sector data! CMD17 ($51) is the command that will return us sectors, it takes to form of <$51,32Bit_Address,$ff>. It's worth knowing that after the CMD0 you don't need to worry about the CRC for the most part. Just send $ff as the lat BIT needs to be 1 (see the SPI diagram above).

After sending the CMD17 command, we wait for the card to return $FE - basically sitting in a loop while it's sending $FF. If we get a value back that isn't $FF or $FE, then it's an error code. Once we DO have $FE we can sit in a loop reading in 512 bytes of data, followed by 2 bytes of CRC (which we'll ignore).

And that's it! This should start getting you real information from the SD card. After this you can just Google FAT16 or FAT32 (which is easier I think), and start to decode the disk. For the sake of completeness I'll include my (inlined and unrolled) byte b = Write8( _byte ) function....

As you can see, once you have the SD card connected to the Pi, there's not actually that much involved in getting data out of it. This makes SD cards ideal for storage on electrical projects - after all, no one says you have to store data in a DOS format, you could just read/write whole sectors and store everything in a custom format. This is what we used to do in the old days, as "standard" formats are bloated for game purposes.

Some point soon, I hope we can release this GameMaker: Studio Raspberry Pi export, then you can have as much fun as I've had with this stuff.  One thing I will say, is that remote development - i.e. not working on the actual machine, is brilliant for this kind of thing, because when you make a mistake and crash the PI (and I do - a lot!) I can reboot it without actually losing my source, or even having to load it all up again! Also, because I don't have to boot into X for dev, it boots really quickly, making these mistakes pretty minor.

All in all, I've had great fun doing this and other little electrical projects. This so far include, my own micro switch Joystick, a 2 line character LCD screen, a 128x64 pixel screen and this SD Card reader. Not bad for a couple of weeks effort!

Here's a list of pages I've found invaluable while doing this project - including some FAT16/32 resources.

SD card stuff

Disk FAT16/FAT32

Friday, October 17, 2014

Why aiming for 60fps or full 1080p is a pointless goal.

So yet again on the web I've seen posts by journalists that 1080p is worthwhile, and that developers should be spending all their time trying to make full 1080p/60fps games, and yet again, it gets my back up...

A few years back I did a post about the mass market, which was an article I did way back in 2004 ( A new direction in gaming), and it seems to me this is the same thing, coming up again.  I mean, seriously, who do you want to buy your games, a handful of journalists, or millions of gamers?

Making a full 1080p/60fps game can be hard - depending on the game of course, but ask professional developers just how much time and effort they spend making a game run at 60fps, while keeping the detail level up, and they'll tell you it's incredibly tricky. You end up leaving a lot of performance on the table because you can't let busy sections - explosions, multiplayer, or just a load of baddies appearing at the wrong time, slow down the game even a fraction, because everyone - no matter who they are, will notice a stutter in movement and gameplay. And that's the crux of the problem. Even a simple driving game, where it's all pretty much on rails, so you know poly counts and the like, has to leave a lot on the table, otherwise if there is a pileup, and there are more cars, particles and debris spread around that you thought there would be, the it'll all slow down, and will look utterly horrible.

Take a simple example. You’re in a race with 5 other cars, cars are in single file, spread out over the track - as the developer expected it to be. You’re in the lead with a clear track. So let’s say the developer had accounted for this, and are using 90% of the machines power for most of the track, and on certain areas, they reduce this to account for more cars and effects - like the start line for example. But suddenly, you lose control, and spin, the car crashes and bits are spread everywhere along with lots of particles. Now the other cars catch up, hit you, and it gets messy. Suddenly, that 10% spare just wasn't enough. it needs a good 30-40% to account for particle mayhem, and damage, and the game slows down. As a gamer, it’s not dropped from 60 to 30 - or perhaps even lower depending on how many cars are in the race (like an F1 race for example). Now, 30fps isn't terrible, and even 20fps would be fine - probably, but the thing is.... the player has experienced the 60fps responsiveness and now its suddenly not handling the same way, and they notice.

The problem is people notice change, even if they don't understand the technical effects or reasons behind it. So going from 60 to 30 will be noticed by everyone, even when you compensate for it. It is much harder to notice going from 30 to 20 when there is frame rate correction going on, but many can still "feel it".

So, if I'm saying you shouldn't do 60fps or full 1080p (if the game struggles to handle it), what should you do? Well, what people really notice, isn't smooth graphics, but smooth input. Games have to be responsive, and it's the lag on controls that everyone really notices. If you move left, and it takes a few frames for that to register - everyone cares, not just hard core gamers. But if a game responds smoothly, then if you’re at 60 or 30 - or even 20! then gamers are much more forgiving. This is mainly because once you're in the thick of the action, they are concentrating on the game, not the pretty graphics or effects. I can prove this too, even to hard core developers/gamers.  Years ago when projectors first came out they were made with reasonably low res LCD panels, and you got what was called the "Screen Door" effect. Pixels - when projected, didn't sit right next to each other, they were spaces between them. Now when you started a film, you'd see this and you would end up seeing nothing BUT the spaces. However as soon as you got into watching and more importantly, enjoying the film, that went away, you never saw the flaws in the display, because you were concentrating on the content.

The same is true of games, sure you power up a game and ooo...look, it’s super high res, and ever so smooth! But 10 minutes later, your deep in the game and couldn't give a crap about the extra pixels, or slightly smoother game, all you care about is the responsiveness of it all. If it moves when you tell it to, your happy.

So what about the 1080p aspect? Well years ago when 1080p started to become the norm, and shops had both 720p and 1080p large screen TVs in store, I happened to be in one, where they had the same model, but one 1080p, and on 720p hanging right next to each other, playing the same demo. I went right up to them, and could still hardly tell the difference. These were 2 50" displays, yet with my noise almost touching them, it was hard to tell where the extra pixels were. Now, if you have a 1080p projector, and are viewing on a 2 to 3-meter-wide screen, I'm pretty sure you'll notice, but on a TV? when your running/driving at high speed - no chance. Every article about resolution in games shows stills, and this is the worst thing to use, as that's not how you play games. It's also worth remembering, that at that moment in time your noting playing, so you’re not concentrating on the game, you’re doing nothing but search for extra pixels, so it's yet another con really.

So what’s the benefit to running slower, at a slightly reduced resolution? Well, 1080p at 30fps means you can draw at least twice as much. That's a LOT of extra graphics you can suddenly draw, and even if you don't need/want to draw that much more, if makes your coding life MUCH simpler. You no longer have to struggle to maintain the frame rate, or worry about when there is a sudden increase in particles or damage - or just more characters on screen.

What about resolution? Well 720p is still really high-res. There are still folk getting HD TV's and watching standard definition TV thinking it's so much sharper than it used to be! The mass market usually doesn't "get" what higher res is until they see it side by side, and once things are moving and they are concentrating on other things, they will neither know, or care.

At 720p, 30fps you can draw over 4 times as much 1080p, 60fps. That's a LOT of extra time and drawing. Imagine how much more detailed you could make a scene with that amount of extra geometry or pixel throughput. Even at 3 times, you could leave yourself a massive amount of spare bandwidth to handle effects and keep gameplay smooth.

2D games "probably" won't suffer too much from this problem, although on mobile web or consoles like the OUYA they will as the chips/tech themselves just isn't very quick.

So rather than spend all your time with tech and struggle to maintain a frame rate most gamers won't notice, shouldn't you spend all your time making a great game instead? If you're depending on pretty pixels to make your game enjoyable, it probably isn't a great game to start with, and you should really fix that.

Games with pretty pixels sell a few copies, truly great games sell far more, and while it's true that games with both will sell even more, the first rule of game development is that only games you release will sell anything at all, and while playing with tech is great fun, constant playing/tuning doesn't ship the game.

Wednesday, September 17, 2014

When Scotland changes forever.....

In a world of history, a truly historic event is rare, but that's what Scotland gets tomorrow. On the 18th of September 2014, we the people of Scotland, get to vote on Independence. We get to decide whether we break away from the United Kingdom, or whether we stay. But either way, things look like they'll change, for the worse or better is anyone's guess.

The Stay together campaign is promising loads of new powers from Westminster, and that the UK will stay strong, in both military and monetary might. They say everything a Yes vote promises is dangerous, and too big a risk to gamble on. Perhaps their right.... perhaps.

Or perhaps not. The Yes campaign obviously states the exact opposite, that staying will continue the downward spiral of Scotland, and more and more power and money going to Westminster. They will continue to take Scotland's Oil and squander it, while giving little or nothing back. They also say the so-called powers that are on offer, will never appear and not to trust them.

A couple of years ago, I started out firmly in the No camp, but over time came over to be a resounding Yes. So what changed my mind?

Well, at the start I was concerned with things like defence, and thought that being an island nation, we can defend our borders better together,  and that leaving would make travel ultra complicated - at least for a few years.

But over time I realised that just like with other nations, partnerships could be done for defence - and probably will be, while travel disruption will actually be fairly minimal as I'm still on a UK passport - that isn't going to change.

The other thing I came to realise was that Scotland is just as capable of running her affairs as Westminster, after all there have been both Scottish Prime Ministers, and Scottish Chancellors. We're a smaller country with amazing natural resources, something Westminster really want to keep a hold of, and I do wonder if it wasn't for Oil and renewable energies (not to mention some place to keep their nuclear arsenal), would they care so much?

I also don't trust the current promise of new powers, because while the "current" leaders may indeed be sincere, there is no way they can promise. Any set of powers MUST go through current parliamentary procedures and be approved by both the House of Parliament, and probably the house of Lords. Back bencher's of all parties - not to mention some grass roots members, have expressed distress - and no small disgust, at additional powers being promised without proper discussion and review. So chances are, whatever comes - IF they come at all, won't be what they are promising, nor what No voters are expecting.

The ironic thing is, that the SNP wanted "Devo-Max" (which is what the offer of new powers is) was on the ballot paper as an offer, but Westminster refused, saying if MUST be a yes/no only. If it was on, I suspect it would have been an easy victory for No/DevoMax folk.

There's also the argument of Money, and what currency to use. I'm indifferent to this at best. We can of course use the English pound, or we could use the Euro or even just make our own. I don't really care either way. I suspect it'll be a few years uncertainty, and then stabilise. Scotland is, or would be, a wealthy nation. Oil rich, great renewable energy, and we export more than we import.

Why does the Better Together campaign think Scotland would struggle? What other Oil rich country struggles? And this isn't even considering the renewables. Scotland is currently on track to have 100% of it's power generated by renewable energy - we're already at 40%.

Not that I actually think we'll win to be honest. While there have been huge - HUGE Yes rally's, I also think those voting No are more the kind to just watch, stay at home... then vote No. I know this because many of my older relatives are no's, and they are exactly like this. On top of this there is also the undecideds, and they may well swing it either way.

But the thing I'm truly excited about, is the level of engagement on this vote. Unlike a general election where the public believes their vote counts for nothing - especially in Scotland, everyone here feels their single vote could swing things, so they HAVE to take part. It's amazing. There has been a 97% registration rate - unheard of in any election, and they are expecting 80-90% of these to actually vote. That's astounding.

So, yes.... I'm looking forward to tomorrow, to take the leap and cast my vote, and while I don't "think" we'll win - I'm certainly not sure, and I'm excited to hear the voice of Scotland - virtually ALL of Scotland.

So I'll vote Yes, and I'll hope everyone else wants to take that leap of faith with me, believe that Scotland is best suited to govern Scotland, because while it might be a tricky road ahead, nothing truly worthwhile is ever easy, and this is something worth taking that leap of faith for, not just for me, but my kids and my kids kids.

Wish us luck, no matter how the vote goes!!