Thursday, 3 April 2014

ZX Sprites

It is a truth universally acknowledged, that as the ZX Spectrum does not have hardware sprites, the programmer is left to concoct her own bitmap graphics routines. It's something every aspiring Spectrum coder should try to do!

Recently me and Marq have been exploring fast and smooth sprite graphics on the 48k Speccy. It's a much discussed topic, yet remains fascinating. Moving graphics can be done in so many ways, and it is not always obvious what to aim for. Games use various tricks to ease the load, and the tricks in turn depend much on what the game is about.

It's still wise to try to build a generally useful sprite engine, before optimizing for different contexts. So far we have managed eight 16x16 masked, freely positioned, flicker-free, smoothly moving sprites, as shown in the above video. (Check also this Youtube video

This first part opens up the basics of the topic. If I get to write an improved version, there will be a Part II.

A 16 x 16 sprite is a common starting point. Any problems related to it can be applied to larger sprites. The above image shows a 32-byte definition of such a sprite. From left to right, top to bottom, the bits would translate to following decimal bytes:


This corresponds to the way the Spectrum stores pixels, one bit per pixel, a byte for eight pixels. The memory-mapped screen is found from the address 16384 onwards. A POKE 16384,255 in BASIC will quickly show this. 

Screen BASICs

The ZX Spectrum display is notoriously a bit disordered, so one of the first issues is to resolve the drawing order. At first glance, it would seem to be enough to increment the drawing address with one byte for each column and 256 bytes for each row. Then, a routine like the one below would render our sprite on screen:

10 LET ADD=17184
20 FOR B=0 TO 15
70 LET ADD=ADD+256
80 NEXT B 
100 DATA 7,224,24,24,32,4,78,2
110 DATA 95,2,159,1,142,1,128,1
120 DATA 128,1,128,1,128,25,64,58
100 DATA 64,50,32,4,24,24,7,224

If only:

Uh oh.
But we're on right track somehow. At least something is happening on screen. As it stands, the address logic changes after every eighth pixel row, as shown in the diagram below. The picture shows in detail the structure of the top left corner of the Spectrum screen, where I tried to draw my sprite. Inside the character row, each pixel row is nicely 256 bytes apart, but the first addresses of each character row are 32 bytes apart.

Furthermore, the screen is divided into three 64-pixel high "slices", each having a start address 2048 bytes apart. Zooming out, the leftmost screen addresses for each character row (every eighth pixel row) are laid out as follows:

This ordering is quite good for drawing 8x8 characters aligned to the character rows. One might even call the Spectrum screen layout a pseudo-character display. But a generic sprite routine has to negotiate the 8-line "boundaries" as well as the two "slice" boundaries. 

The BASIC program below identifies the slice block and character rows for a given vertical pixel coordinate, and draws the "sprite" accordingly.

10 FOR S=0 TO 15
20 LET Y=11+S
50 LET YR=Y-(CROW*8)
70 LET ADD=16384+BLOCK*2048+CROW*32+YR*256
100 NEXT S
110 DATA 7,224,24,24,32,4,78,2
120 DATA 95,2,159,1,142,1,128,1
130 DATA 128,1,128,1,128,25,64,58
140 DATA 64,50,32,4,24,24,7,224

OK, indeed
The object described here hardly deserves the name "sprite". BASIC is simply too slow, and there's lot more to do than just laying the bits on screen. Proper sprite routines can only really be done in machine code.

Even then, it's possible to improve the above listing, by calculating the addresses beforehand into a table. It would be a bit silly to calculate the address for each sprite row.

20 DIM A(176)
30 FOR Y=0 TO 175
60 LET YR=Y-(CROW*8)
80 LET ADD=16384+BLOCK*2048+CROW*32+YR*256
90 LET A(Y+1)=ADD
100 NEXT Y
120 RESTORE 180
130 FOR I=0 TO 15
150 POKE A(11+N),L: POKE A(11+N)+1,R
160 NEXT I
170 DATA 7,224,24,24,32,4,78,2
180 DATA 95,2,159,1,142,1,128,1
190 DATA 128,1,128,1,128,25,64,58
200 DATA 64,50,32,4,24,24,7,224

At least in this respect, writing sprite routines in assembler is not that different. It's about finding ways to offload the burden from the drawing parts. This is why many fast routines use a buffered drawing of some sort.

Sprite shifting

The BASIC listings simplified many things. Much has been said about the vertical coordinates. How about the horizontal? Fiddling with the BASIC listings above would show that adding one to the sprite position address moves the sprite one character column to the right. This at least is straightforward, but it is not smooth. There are ways to recalculate the sprite graphics on the fly, but pre-shifted sprites are easier and faster.

A 16x16 sprite GFX and its' mask, with room for shifting.
Shifted 16x16 sprites are in reality 24x16 sprites, and each graphic needs 8 shifted variants. This way they can be aligned horizontally with the character columns. The downside is that it takes more memory. 

All the sprite variants also need a mask, which is an inversion of the area that will be cleared before drawing the proper sprite. This way the sprites can be drawn over a background without showing any of the pixel background through the sprite, or producing other ill effects. Many games get away without using masks or backgrounds, and can even look better for it (think Dan Dare), but such an approach is hardly generic.

The machine code routines discussed below use shifted sprites.

Sprites in z80 machine code

Smooth sprite movement has to be tied to the screen update cycle, which is refreshed 50/60 times a second. In machine code, the HALT instruction tells the processor to wait until the refreshing beam has returned to the top of the screen.

After this, the beam travels right, returns to the left side of the screen and travels down, refreshing the screen during the process. To avoid flickering and glitches, changes to the screen should be done before the beam hits the drawing area.

So, a conventional sprite-drawing program might follow this order:

The z80 running at 3.5mhz can do quite a bit before the beam is arrives at the pixel area. (Or "the display file" as it is called in the official Spectrum manuals.)

Of course, even when the beam is in the pixel area, things can be drawn below the current beam position, and the results will still be smooth. Many scene demo effects take advantage of "racing the beam" in this way. Even a sprite routine may benefit from such a scheme, but for a generic routine the drawing is safest to do before the beam arrives at the pixel area.

The sprite program shown in the video follows the order below. The border colours are not just for fun, but for showing what actions are being taken at that scanline position. The routine draws the sprites into a hidden buffer screen, and uses a fast copy routine to do bring them on the visible screen. An address table holds the position information for the fast copy routine.

  • Copy the entire background image to the entire buffer
  • Draw sprites to buffer and create a new address table for them
  1. Wait Vertical Blank (=HALT instruction)
  2. RED:   Copy bytes to screen from buffer, using the sprite address table
  3. BLUE:  Copy background to sprite positions at the buffer, using the sprite address table
  4. YELLOW:Change sprite coordinates
  5. BLUE:  Store the address table
  6. YELLOW:Draw sprites to buffer and create a new address table for them
  7. BLACK: Wait until the beam is outside the pixel area (estimated)
  8. RED:   Copy background to sprite positions on the screen, using the stored address table
The sprites are drawn when the beam is on its "way in" to the pixel area, and wiped out on the way out. It is a bit unelegant to have to wait at the end, as the waiting time depends on the computer speed. So, this solution is fixed for the original 48k Spectrum timing.

Note that here the sprites are drawn well before the beam arrives at the pixel area. The fast copy routine could even move 12 sprites to the screen. Yet drawing more than 8 sprites altogether (buffer, clearing) is not yet possible. It's easy to see though, that there are ways to reduce the load, especially the needless duplicating of the address table.

Buffered drawing

In this sprite routine, there are three "screens": The real display screen, a source background image and the drawing buffer. In addition, the sprite graphics need to be stored somewhere. 

The diagram below shows the Phase 6 in the above list. (The second "yellow" portion.) Here we draw the sprite to the invisible buffer screen and store its screen position as an address into the sprite address table for later copying.

The actions required for drawing a single byte are highlighted. The diagram only shows the part relevant for the sprite, which in this case is drawn to the top left corner of the buffer, waiting to be moved on the same relative screen position. The mask and graphics are interleaved for purposes which is explained later.

Phase 6: The diagram illustrates how the sprite is drawn into the buffer screen.
Each time the sprites are drawn to the buffer, an address table is also renewed. The address table is at the heart of the fast drawing routine (phase 2). One sprite has 16 rows, so drawing a sprite to the buffer also writes 16 addresses to the table. The address table points to the locations in the drawing buffer, from which the sprites are copied after the HALT:

Phase 2. Fast copy from the buffer to the screen
This means a lot of the calculations needed for computing the outcome (such as the row boundary calculations and combining the mask/background) are done in hiding, whereas the fast copy is only concerned with moving bytes directly between the buffers and screen areas. 

In z80 assembler, the fast copy might look something like this:

ld sp,#addresstable
pop hl ; row 1
ld d,h
ld e,l
res 7,d
pop hl ; row 2
ld d,h
ld e,l
res 7,d
pop hl ; row 3
ld d,h
ld e,l
res 7,d

[16 rows for each sprite]

; Just an example, a sprite in the top left corner
.dw #0XC000,#0XC100,#0XC200,#0XC300,#0XC400,#0XC500,#0XC600,#0XC700
.dw #0XC020,#0XC120,#0XC220,#0XC320,#0XC420,#0XC520,#0XC620,#0XC720

The stack pointer is placed at the beginning of the table, and the addresses are loaded in the HL register using POP HL, which also increments the stack pointer by two. The LDI performs the equivalent of LD (DE),(HL), moving the contents from address at HL to the address at DL, incrementing both DE and HL in the process. For 24-pixel wide sprites, three LDIs are required for one row.

The table could have from-to address pairs, but writing both addresses during buffer drawing proved to be a bit cumbersome. So the "destination" address DE is constructed out of the "from" address HL by altering the high byte. This is a tiny bit slower than POP:ing both from the table, but it also keeps the table shorter for copying purposes. (phase 5)

This address shifting also means the buffer locations cannot be freely chosen. 0XC000 (49152) and the screen address 0X4000 (16384) are in a good relation to each other, as only one bit needs to be changed between them. There are three versions of the above routine, all different depending whether it is about BUFFER->SCREEN, BACKGROUND->BUFFER or BACKGROUND->SCREEN copying. 

The lines

Let's get back to where we started: The ZX Spectrum screen order. Despite the fast copying routines, the sprites need to be drawn into the buffer, and at least somewhere during the process, the line order needs to be negotiated. It's no good if the buffer drawing routines, even if hidden, are too slow.

The overall assembler source is a bit too daunting to publish here, as it is largely a Processing-generated bundle of tables and repetitive code, held together with some C. So I'll stick to explaining the overall idea and some of the more interesting points.

Obviously there has to be a table that contains the drawing address for each vertical pixel row. But accessing this table 16 times each time a sprite is drawn would be less than optimal. This can be avoided. But if one wants to avoid conditional jumps (and one wants to avoid them) there has to be a number of variants of the routine, depending on which pixel row the sprite is drawn to.

With eight sprites, any commands added to the pixel drawing order gets repeated not only 8 times, but 128 times or more! So there's a great incentive to remove slow code from the heart of the buffer drawing.

It would be neat to just have a full sprite drawing routine for each vertical pixel row. This would produce 175x16 copies of the sprite row drawing code, which takes far too much memory. The amount can be reduced: there are only 23 ways how a 16-pixel high sprite drawing might unfold. This would produce "only" 23x16 times the row drawing code. It doesn't sound much but still it's more than 20 kilobytes, a bit too much.

What we did was a bit nasty: self modifying code. One pixel row drawing code is repeated only 39 times. Each variant has it's own labeled entry point. When the sprite drawing is invoked, the program jumps into the relevant entry point, depending on the sprite vertical coordinate. (Yet another table) The diagram below describes the whole code portion and an example case:

The example sprite is to be drawn at Y coordinate 20. A vertical address table tells that this row uses the variant 4. The NOP (0x00) instruction at the address of EXIT4: label is overwritten with a RET (0xC9) instruction. HL is loaded with the label ENTRY4: address. JMP (HL) takes the program counter there.

The routine then draws four rows, skips the character boundary, draws another eight rows, skips another character boundary, draws four more rows and exits the drawing routine. The RET is overwritten with a NOP.

There have to be rewritten exit points, because only 16 rows are needed, and we want to avoid conditional jumps and wasting registers on counters. So, drawing sprite variant 0 means jumping to the entry label 0, while writing a RET to the EXIT0:. Jumping to entry 4 means writing a RET to the exit point 4 and so on.

The code below describes what happens within the "draw row" portion in the above diagram.

; stack is pointed to the beginning of sprite graphics
; (mask and gfx interleaved)
; de holds the drawing address
; jp(hl) brings the program counter here
; bc holds the beginning of the address table


; de is written to the address table

      ld a,e      
      ld (bc),a
      inc bc
      ld a,d
      ld (bc),a
      inc bc

; draw one sprite row to the buffer (3 bytes wide)
; stack handily gives both the mask and the graphic byte

      ld a,(de)  ;get buffer byte
      pop hl     ;get mask and gfx
      and a,l
      or a,h
      ld (de),a  ;draw to buffer
      inc e      ;right

      ld a,(de)  ;get buffer byte
      pop hl     ;get mask and gfx
      and a,l
      or a,h
      ld (de),a  ;draw to buffer
      inc e      ;right

      ld a,(de)  ;get buffer byte
      pop hl     ;get mask and gfx
      and a,l
      or a,h
      ld (de),a  ;draw to buffer

      nop        ; may be overwritten with ret

After the pixel row, the destination address is adjusted for the next pixel row.

; normal
      dec e      ;left
      dec e      ;left
      inc d      ;down

If the next address crosses the character row, this will be used instead:

; pass the character row boundary
      ld a,e
      add a,#30
      ld e,a
      ld a,d
      sub a,#7
      ld d,a

Or, finally, if the next pixel row is beyond the slice block boundary, this is needed:

; pass the slice block boundary
      inc d
      ld a,e
      add a,#30
      ld e,a

That's it for the time being. We think this routine can still be significantly improved. Perhaps the nasty self-writing can be avoided. Perhaps it might be possible to get rid of the un-elegant wipe at the end of the screen. There are some wild ideas brewing, but better not boast about them before they are real...

SDCC Small Device C Compiler.
FUSE The Free Unix Spectrum Emulator.

Smith, Chris. (2010) The ZX Spectrum ULA: How to design a microcomputer. ZX Design and media.
Smith, Derek M. (2004) Sprite Graphics Tutorial. Accessed from
Vickers, Steven (1982) ZX Spectrum BASIC programming. Sinclair research.
Zaks, Rodnay (1982) Programming the Z80. Third revised edition. Sybex.


  1. I still believe that a vertical linear 256x256 pixel offscreen buffer would be the fastest option :) Screen refresh needs to be optimized so that there's no need for a separate wipe/draw cycle.

  2. Awesome article!

    Thanks very much - this helped me out a load!

  3. Having a good look through this article and very impressed. One point I would make is that you could remove the two DEC E's in the sprite routine by simply storing your sprite imagines in a ZIG ZAG format (hope that makes sense).