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Revision as of 05:18, 6 July 2020 by Superogue (talk | contribs) (Registers)

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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:

Z80 for X86 programmers

Z80 can be seen as the little 8bit 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
  • 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
  • HL = Can be seen as BX (H=BH,L=BL) or SI in a (SI) setting, like BX also used for adressing.
  • IX = 16 bit Index Register X, can also be accessed with IXH,IXL
  • IX = 16 bit Index Register Y, can also be accessed with IYH,IYL


Here is a rough translation for some of the Z80 instructions:

  • BIT = TEST
  • CP = CMP
  • DJNZ = LOOP (decreass B and checks not zero)
  • EX = XCHG
  • EXE = Exchange all registers with Shadow registers, can be used a bit like PUSHA/POPA
  • HALT = HLT
  • JP = JMP
  • JR = JMP NEAR (Jump Relative)
  • LD = MOV

ZX Spectrum

The ZX Spectrum consists of a Z80A @ 3.5 MHz CPU with either 16k, 48k or 128K of RAM.

Setting up

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.

Start values

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 diplay

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.

ScreenPosition = (page<<11) + (character row<<8) + (subrow<<5)

Adding Color

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

You can set the border color to any of the 8 colors with:

out (254),color

Getting something on screen

To be added soon.


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

To be added soon.

Make some noise - AY

To be added soon.

Additional Resources

I found resources on ZX Spectrum sizecoding to be sparse.