ZX Spectrum Developer: 2009

I have always been a big fan of large scrolling map games, where the play area is much larger than the screen. Sonic the hedgehog is a good example. I suppose even Lunar Jet man could be classed as one of this games.

I then played a game on the spectrum called “Maze Mania” by Hewson Consultants and thought “How did they do that?”.

Discussion

There is an “animated” thread discussing this here: http://www.worldofspectrum.org/forums/showthread.php?t=24788

Basically, I observed that the main screen is 28 chars by 16 chars in size, moving 1 char (8 pixels) in all directions. Moving 1 char avoided any colour clash and looked really good. The game ran at 12.5 Frames per second, and looked pretty smooth.

Investigating the code using a disassembler I noticed

1. They drew the tiles to a back buffer
2. Drew the sprites in their correct positions over the top of the back buffer tiles
3. Copied the buffer to the main screen

The following diagram showed how I planned to implement this (I was aiming for 24x16) where Diagram 1 is the back buffer, 2 is the region to copy and 3 is the end result on the screen.

Back buffer

Region To Copy

Display

Copying the Back Buffer

In Maze Mania, in order to copy the back buffer to the screen I think they have a table for each of the lines (18 x 8 = 144 lines) containing the source address and the destination screen address. The SP is pointed to this table, and the values are "popped" into HL and DE. Then 28 x LDI's are done to copy from the source to the destination.

Here is my suggested code based on 16 rows (128) x 24 columns

HALT            ; Wait for Top Of Scanline, in reality would set up IM2 or something
DI              ; Disable Interrupts
LD (spstore),SP ; Store Stack Pointer
LD SP,table_of_source_and_dest_addresses ; Point SP to our table of source and destination (display) addresses

; Wait for scan line
LD BC,$0320 ; 800
loop:
DEC BC
LD A,B
OR C
JR NZ,loop

LD A,$80 ; 128 (16 Rows of 8 bytes)
line_loop:
POP DE   ; Get DESTINATION (Display) from our table
POP HL   ; Get SOURCE address from our table
LDI        ; Copy Contents of (HL) to (DE) 24 times (24 columns)
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI
LDI

DEC A    ; Move to next row
JR NZ,line_loop ; repeat until all done

LD SP,(spstore) ; restore Stack Pointer
EI ; enable interrupts

An example of the "table_of_source_and_dest_addresses" then looks like this:

Code:

table_of_source_and_dest_addresses:
; Destination (Display), Source
defw $4000,$E500,%4100,$E600..... for all 144 lines (18 Rows)

Where the first 2 bytes are the screen destination, the next 2 are the back buffer (in this case starting at $E500)

This takes about 140,000 T-Cycles, so I reckon with some optimised Tile Writing and Sprite Code I could get 50/4 = 12.5 Frames per second, or at least thats my goal.

Writing Tiles

We need to write tiles to the back buffer. For starters, I have written code that writes them straight to the display.

The code reads from a map, converts to a graphics address and writes the tile to the display. The map is 16 x 16 squares (256). The Clear square is Tile 0, the Lines are Tile 1 and the solid block is Tile 3.

The following code when assembled will produce the following:

If you move the screen address around (de), or the map position (bc) you will see the tiles react accordingly.

; draw03.asm

; Draw a 4x4 block and fill the screen

; de = Screen address

; Takes about 100,000 cycles to do this (8 columns * 4)

org 08000h

; Turn border black

xor a ; a = 0

out (254),a ; send out of port to set border

di ; disable interrupts

ld de,04000h ; de points to screen (top left) ld bc,map ; bc points to map position (0,0)

ld a,5 ; 5 Rows x 4 = 24 rows

loop1: push af ; store counter push de ; store screen address (First column)

ld a,8 ; 8 columns to draw

loop2: push af ; store column counter

push de ; store screen address

ld a,(bc) ; Get Map TILE at map Position

ld l,a ; e = tile

ld h,0

; Multiply by 128 to get tile gfx address

rl l

rl h

rl l rl h

rl l

rl h

; add address to base gfx address

ld a,h add a,gfx/256

ld h,a

push bc ; store map position

call drawblock ; draw the block

pop bc ; get map position back

inc c ; increase map position (column)

pop de ; retrieve screen address

inc e ; Move screen address along 4 characters to next column

inc e

inc e

inc e

pop af ; get column counter

dec a ; and decrease

jr nz,loop2 ; repeat until all columns drawn

pop de ; get original (first column) screen address back

ld a,80h ; 4 lines (32 * 4 = 128) add a,e ; add to screen address (e) ld e,a

jr nc,loop3 ; If we have not crossed the 256 byte border, ok to draw

ld a,d ; Move to next 2048k screen third

add a,8 ; By adding 8 to d, 001000 = 2048

ld d,a

loop3:

ld a,c ; We have drawn 8 columns so move to next row (16 by 16 map) add a,8 ; add 8 columns to get to next row ld c,a

pop af ; get counter back

dec a ; decrease counter

jr nz,loop1 ; repeat if we are not at the bottom row

ret

drawblock:

; Start

ld (spstore),sp ; Store Stack Pointer

ld sp,hl ; SP = Graphics Block

ex de,hl ; hl now has screen address

ld a,4 ; 4 Rows of Chars

a1:

; Store Counter

ex af,af'

;-------------------------------------

;This code is repeated 8 times

; Get 4 bytes from GFX

pop de

pop bc

; Store in screen position

ld (hl),e

inc l

ld (hl),d

inc l

ld (hl),c

inc l

ld (hl),b

; Next screen line

inc h

; Get 4 more and work back

pop de

pop bc

ld (hl),b

dec l

ld (hl),c

dec l

ld (hl),d

dec l

ld (hl),e

inc h

;-------------------------------------

pop de

pop bc

ld (hl),e

inc l

ld (hl),d

inc l

ld (hl),c

inc l

ld (hl),b

inc h

pop de

pop bc

ld (hl),b

dec l

ld (hl),c

dec l

ld (hl),d

dec l

ld (hl),e

inc h

pop de

pop bc

ld (hl),e

inc l

ld (hl),d

inc l

ld (hl),c

inc l

ld (hl),b

inc h

pop de

pop bc

ld (hl),b

dec l

ld (hl),c

dec l

ld (hl),d

dec l

ld (hl),e

inc h

pop de

pop bc

ld (hl),e

inc l

ld (hl),d

inc l

ld (hl),c

inc l

ld (hl),b

inc h

pop de

pop bc

ld (hl),b

dec l

ld (hl),c

dec l

ld (hl),d

dec l

ld (hl),e

inc h

; Work out next screen line

NextPixelLine:

ld a,h

and 7

jr nz,npl_ok

ld a,h

sub 8

ld h,a

Next_Char_Line: ld a,l

add a,32

ld l,a

and 224

jr nz,npl_ok

ld a,h

add a,8

ld h,a

; loop

npl_ok:

ex af,af'

dec a

jr nz,a1

; restore SP and return

ld sp,(spstore)

ret

spstore: defw 0000

org 0e400h

map:

defb 1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 1,1,1,2,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,2,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,2,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

defb 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

org 0e500h

gfx:

defb %11111111,%11111111,%11111111,%11111111

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defb %10000000,000000,000000,000001

defs 128,128

defs 128,255

end 08000h

Conclusion

So far, we have worked out in theory how the map will work and have developed code to

1. Copy the back buffer to the screen
2. Draw tiles to the back buffer (The above code is direct to the screen)

The next step (Part 2 to follow) will add colour to the tiles and draw attributes as well as left, right, up and down controls to allow you to scroll through the map.

Further parts to this series will then draw masked sprites on top of the background.

My goal is to document the complete development of this game, so stay posted and constructive comments are always welcome.

You can download the source and “tap” image for the spectrum for this article here

*** ADDITION ***

You can download a work in progress of step 2 here. Use Q,A,O & P to navigate the map.

When I develop Machine Code games for the Sinclair ZX Spectrum, I use the following Development Tools:

ConText Text Editor (http://www.contexteditor.org/)
Pasmo Assembler (Pasmo)
Spectaculator Emulator (http://www.spectaculator.com/)

Installing “Spectaculator”

Download and Install “Spectaculator” 30 Day Trial. I cannot recommend this emulator more highly, it is awesome

Installing “Context” Editor

The Editor allows me to write the code with Syntax Highlighting, as well as define events for key presses that assist me in running and testing my programs.

Download the editor (1.6 MB) from http://www.contexteditor.org/ andfollow the installation instructions:

Step 1:

Step 2:

And follow the wizard until it has installed

Install “Pasmo” Assembler

Next, download the Assembler from http://www.arrakis.es/~ninsesabe/pasmo/

I use the 0.5.3 version, downloadable as a ZIP from pasmo-0.5.3.zip

Once downloaded, Unzip and extract.

Install in a directory of your choice

Configure Context

From the menu choose “Options -> Environment”. A dialog box will appear.
Click the “Execute Keys” tab.
Click the “Add” button, and add an entry for files with a “asm” extension.

Set up the “F9” Functionality.

This will allow the file to be compiled, and a “TAP” file which includes a BASIC loader will be created.

Execute: Browse To and Select the Pasmo executable.
Start in: %p (File path)
Parameters: --tapbas %n %F.tap
- This will create a ZX Spectrum TAP file with BASIC loader, output to same filename with “TAP” extension.
Use short DOS names for Pasmo.
Output to compiler window.
Scroll to last line for verbose output.

Set up the “F10” functionality

This will execute “Spectaculator” and pass in the compiled filename.

Execute: Points to the Spectaculator executable.
Start in: %p (File path)
Parameters: %p\%F.tap
- This means Path, and Filename (tap)
Use short DOS names for Emulator

Testing the Development Environment

Create the following test program in Context, and save to your project folder as “text.asm”.

org 08000h
ld hl,04000h
ld (hl),%10101010
ld de,04001h
ld bc,17FFh
ldir
ret
end 08000h

Press “F9” and the program should assemble.

Press “F10” to execute “Spectaculator” and you should get the following results:

Congratulations! You are now ready to develop Machine Code on your Windows PC!

ZX Spectrum Developer

Tuesday, 25 August 2009

Scrolling Map Project (Update)

Friday, 8 May 2009

Scrolling Map Project (Part 1)

Discussion

Copying the Back Buffer

Writing Tiles

Thursday, 29 January 2009

Fast Sprites using Pre-Shifted Data

Advantages

Timing

Colour

Download

Next Steps

Wednesday, 28 January 2009

Setting up the Development Environment on a Windows PC

Installing “Spectaculator”

Installing “Context” Editor

Install “Pasmo” Assembler

Configure Context

Set up the “F9” Functionality.

Set up the “F10” functionality

Testing the Development Environment

Contributors

Followers