Z80 based CPUs
- 1 Introduction
- 2 ZX Spectrum
- 3 Amstrad CPC
Wanting to start sizecoding on a Z80 platform in this day and age can be tough.
So here is a bit of help to get you started:
The Z80 can be seen as the little 8-bit brother of X86 chipsets, with many similarities. If you are coming from a X86 background, this might help you get a bit more grip on the Z80. These are the register pairs of the Z80, as seen from a X86 programmers perspective.
- AF = AL + Flags
- HL = Can be seen as BX (H=BH,L=BL) or SI in a (HL) setting, like BX also used for adressing.
- BC = Can be seen as CX (B=CH,C=CL), often used for loops
- DE = Can be seen as DX (D=DH,E=DL) or DI in a (DE) setting
- IX = 16 bit Index Register X, can also be accessed with IXH,IXL
- IY = 16 bit Index Register Y, can also be accessed with IYH,IYL
For each register, there also exists a shadow register that can be swapped out by using the EX instruction.
Note: For a lot of operations, you can only use the A(8bit) and HL(16bit) registers. The Sjasmplus assembler doesn't really do proper syntax checking for this so beware.
Here is a rough translation for some of the Z80 instructions:
- BIT = TEST
- CP = CMP (although the Z80 has many other handy compare functionality)
- DJNZ = LOOP (decreases B and checks not zero)
- EXE = Exchange all registers with Shadow registers, can be used a bit like PUSHA/POPA
- EX = XCHG
- HALT = HLT
- JP = JMP
- JR = JMP NEAR (Jump Relative)
- LD = MOV
- LDI = MOVSB (tmp=(HL),(DE)=tmp, DE++, HL++)
- LDIR = REP MOVSB (tmp=(HL),(DE)=tmp, DE++, HL++, BC--)
Learning Z80 Assembler
There are many Z80 tutorials available online, but one i found very simple and clear is at this 1996 styled webpage ;-)
There is no proper index-page for this, which is why i linked all the lessons above, but you can continue to the next lesson by clicking at the next lesson at the bottom of the page.
Also, here is a compact 'cheat sheet' with some basics for various Z80 systems: https://www.chibiakumas.com/z80/CheatSheet.pdf
The ZX Spectrum consists of a Z80A @ 3.5 MHz CPU with either 16k, 48k or 128K of RAM.
Setting up your development platform for the ZX Spectrum is quite easy, first get the following tools:
- Assembler: SJASMPLUS -This assembler has nice macros for creating Binaries and SNA snapshot files out of the box. You can download it at https://sourceforge.net/projects/sjasmplus/
- Emulator(s): I Found FUSE, UnrealSpeccy and EightyOne to work best for my usecase. Most emulators can read TAP, SNA and TRD files out of the box.
Upon startup (when called from basic), the following values can assumed:
- The alternate HL register is set to 0x2758
- BC = start address
- A = C
Video display on the ZX Spectrum is mostly CPU based with little hardware features. No hardware sprites, no specific text or video modes, only a 256x192 byte screenbuffer with 1bit pixeldata located at $4000 in memory. It is ordened a bit strange in 3 sections of 256x64 pixels, then character rows, then subrows.
Address = 010RRLLL RRRCCCCC
- where RRRRR is the row number (0..23)
- CCCCC is the column number (0..31)
- LLL is the line number within the cell (0..7)
The ZX Spectrum has a 32x24 colormap located at $5800 where you can write color information for each 8x8 tile. It has has 8 colors (INK and PAPER) with 2 brightness settings that can be set like this.
color = brightness(64) | (PAPER<<3) | INK
Because updating pixel memory can be slow, especially when you are grasping for bytes, some of the tiny intros on the zx spectrum prefer to use the colorram for the effect, sometimes in combination with an overlaying pattern.
You can set the border color to any of the 8 colors with:
Getting something on screen
Now to get something on screen, lets fill our colorram with a simple AND pattern, like so:
ld de,5800h ld b,24 yloop: ld c,32 xloop: ld a,c and b and 7 ; make sure range is 0..7 ld (de),a inc de dec c jr nz,xloop djnz yloop
Using a Backbuffer
While the above code will run fine, you might want to consider using a backbuffer for more complex stuff. You can then simply write to another adress define by BACKBUFFER (for example A000) and copy the buffer to colorram like so:
halt ; synchronize ld hl,BACKBUFFER ld de,5800h ld bc,768 ldir
Overlaying simple graphics
If you don't want to use a solid color/tile for. You could copy a single tile across the screen at startup for some flair.
ld a,0x55 ; 01010101 pattern ld bc,0x1800 copyloop: ld (de),a inc de dec c jr nz,copyloop djnz copyloop
Alternatively you could generate a pattern using logic operations or random noise/data.
The original Spectrum has only a 1 bit sound capability (BEEP) through its internal speaker. Later models included the AY-3-8910 Soundchip which provides 3 channels of PSG sound.
Make some noise - Beeper
Setting/toggling bit 4 of port 254 will enable the beeper speaker.
ld a,$10 ;Bit 4 set beeploop: xor $10 ;toggle Bit 4 out (254),a ld b,90 ;wait djnz $ jp beeploop
Make some noise - AY
You can access the AY soundchip be outputting to the following ports:
ld bc,0xfffd ld a, ay register number out (c),a ld b,0xbf ld a, data byte out (c),a
For more information about the soundchip, check out: https://www.atarimagazines.com/v4n7/stsound.html
I found resources on ZX Spectrum sizecoding to be sparse.
- All kinds of Z80 Information http://www.z80.info/index.htm
- Assembler subforum on world of spectrum https://worldofspectrum.org/forums/categories/assembler
- Blogpost on ZX Spectrum coding from a X86 coder's perspectove on superogue's sizecdoing blog (soon)
- 128byte/256 byte zx spectrum productions by goblinish https://www.pouet.net/groups.php?which=11696&order=type
- 128byte/256 byte zx spectrum productions by gasman https://www.pouet.net/groups.php?which=1218&order=type
The Amstrad consists of a Z80A @ 3.5 MHz CPU
- Assembler: -
- Emulator(s): -
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