ZX Spectrum

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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:

  • SjASMPlus (z00m's fork) - cross-platform open source assembler with nice macros for creating Binaries, TAP, TRD and SNA snapshot files out of the box, embedded Lua scripting and support for similar CPUs (Z80/R800/Z80N/i8080/LR35902).
  • Rasm - extremely fast cross-platform Z80 assembler with many features
  • Pasmo - for the .TAP create. Available at : http://pasmo.speccy.org
  • Emulator(s): I Found ZX Sping, FUSE, UnrealSpeccy and EightyOne to work best for my usecase. Most emulators can read TAP, SNA and TRD files out of the box.

Alternatively, you can use the online Bazematic tool to do a lot of the early development at bazematic.demozoo.org

Start values

Upon startup (when called from basic), the following values can assumed:

  • The alternate HL register is set to 0x2758
  • IY = 0x5c3a
  • 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

Memory Configuration

128K separated to 8 pages (16384 bytes size) Default configuration is:

page 5: $4000-$7fff
page 2: $8000-$Bfff
page 0: $C000-$Ffff

There are two screens - page 7 and page 5. port $7FFD allow to control memory and screens:

bits 0-2 - page number mapped to memory at $C0000
bit 3 - Select page 5(0) or page 7(1) to be displayed.
bit 4 - ROM Select. 0-128K,1-48K

Example of use 

loop: ei
 halt
pg: ld a,$17
 ld bc,$7ffd
 out (c),a
 xor $0A
 ld (pg+1),a
... do something with $C000-$DB00
 jp loop

$17 is use 48K, map page 7 After xor $0A we'll get value $1D(use 48K ROM, display screen at page 7 and map page5 at $C000).This is so-called "double buffering". See also : https://worldofspectrum.org/faq/reference/128kreference.htm

Graphics

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 predefined data, logic operations or random noise/data.

ROM Routines

calculate Pixel coordinate 

;IN: A=X coordinate,C=L=Y coordinate
;OUT: HL=screen address,A=bit value
call #22B0



;write pixel
 CALL  8933 ; C=X(0..255),B=Y(0..175),5C7D=COORDS


Other Useful routines

;calculate address of next line,HL=address
down_hl:
 INC h
 LD A,h
 AND 7
 RET NZ
 LD A,L
 ADD A,#20
 LD L,A
 RET C
 LD A,H
 SUB 8
 LD H,A
 RET

;move up at screen, HL=address
up_hl
     LD A,H
     DEC H
     AND 7
     ret nz
     LD A,L
     SUB 32
     LD L,A
     ret c
     LD A,H
     ADD A,8
     LD H,A
 ret

;Calculate vertical line screen Address, IN : A=Y coordinate, OUT : HL=address
py2saddr:
        ld l,a
;	ld a,l
	and $07
	ld h,a
	ld a,l
	and $c0
	rra
	inc a
	rrca
	rrca
	or h
	ld h,a
	ld a,l
	add a
	add a
	and $e0
	ld l,a

;Calculate Attribute address, IB: reg D=Y,reg E=X, OUT: HL=address, destroys DE
 ld l,d
 add hl,hl
 add hl,hl
 ld h,$11
 add hl,hl
 add hl,hl
 add hl,hl
 ld d,0
 add hl,de

or alter version 

;H=Y coordinate,L=X coordinate
	ld a,h
	rrca
	rrca
	rrca
	push af
	and 3
	add a,$58
	ld h,a
	pop af
	and %11100000
	add a,l
	ld l,a


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 by 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

Additional Resources

I found resources on ZX Spectrum sizecoding to be sparse.

Small demos with documented source code

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