Z80 based CPUs

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Introduction

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:

Registers

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 addressing.
  • 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 of the main registers there also exists a shadow register. These cannot be accessed directly, but must be swapped in and out with the main register set. The shadow registers are usually denoted by the ' symbol. They can be swapped with the following commands:

  • EX AF,AF' = Swaps AF with AF'
  • EXX = Swaps BC, DE and HL with BC', DE' and HL'

There are no shadow registers for the index registers.

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.

Instructions

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


ZX Spectrum

The ZX Spectrum consists of a Z80A @ 3.5 MHz CPU with either 16k, 48k or 128K of RAM. Most demos are targeted at the Spectrum 128 because it includes more memory, a shadow screen buffer and an AY soundchip. The different models have slightly different timings - this will cause issues if you are doing cycle-exact effects like multi-color.

Setting up

Setting up your development platform for the ZX Spectrum is quite easy, first get the following tools:

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 display

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)

Calculating a screen address from XY coordinates is complicated due to the weird screen layout. In a larger demo you would generate a lookup table - it's usually best to avoid such calculations in small demos, but it can be done in under 30 bytes, eg: http://www.breakintoprogram.co.uk/computers/zx-spectrum/screen-memory-layout

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

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.

Border Color

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

ld a,0 ; bottom three bits of a contain the color
out (254),a

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

Another alternative method is to use the 128's memory paging, which provides a second screen buffer. This buffer is located at a different memeory location, but otherwise it is the same:

	; main loop starts here
	ld	a,00010111b	; set up memory banks and screen here
mainloop
	halt			; sync
	xor	00001010b	; flip screens
	out	($fd),a
	push	af

	; render code goes here
	; screen buffer is location at 0xC000 instead of 0x4000

	pop	af
	jr	mainloop

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.

Sound

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

Note that the bottom three bits of port 254 also set the border color (see above).

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

Small demos with documented source code

Additional Resources

I found resources on ZX Spectrum sizecoding to be sparse.

Amstrad CPC

The Amstrad consists of a Z80A @ 3.5 MHz CPU

Setting up

Setting up..

  • Assembler: -
  • Emulator(s): -

Video Display

No information yet

Sound

No information yet

Additional Resources

No information yet