ZX Spectrum Assembly, Space Battle – 0x01 Definition of graphics
In this chapter of ZX Spectrum Assembly, we will define all the graphics used in Space Battle.
Translation by Felipe Monge Corbalán
Table of contents
Definition of graphics
We will learn how to do the hexadecimal/binary conversion on the fly by doing our first exercise.
You can use ZX Paintbrush, or you can use the templates I created for GIMP, which you can download from these addresses:
The templates I have prepared are as follows:
- 8x8Template: for creating 8×8 pixel graphics.
- 256x192Template: the size of the ZX Spectrum’s screen.
For Space Battle we will use 8×8 templates. I will show you the image of the graphics, the hexadecimal codes, and your job will be to convert the codes into pixels, head first, to draw the graphics on the templates.
Hexadecimal/binary conversion
Although at first it may seem complicated to convert a hexadecimal number to binary, and vice versa, it is very simple and practically straightforward. We need to know the value of each bit (pixel) in blocks of four, which gives us a value between 0 and F, a value that can be represented by each hexadecimal digit.
In a byte, each bit to one has the following values:
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
When we convert from hexadecimal to binary, we divide the byte into two blocks of four bits (nibbles), resulting in a range of values between 0 and F (8 + 4 + 2 + 1 = 15 = F). To convert from binary to hexadecimal, we add the value of the bits to one of each nibble, which gives us the value in hexadecimal.
Suppose we have the following binary value:
0101 1001
If we add up the values of the nibbles, the result would be:
0 + 4 + 0 + 1 = 5 8 + 0 + 0 + 1 = 9
The result is that 01011001 is 59 in hexadecimal.
In hexadecimal a byte is represented by two digits. What if the value of some of the nibbles is greater than nine?
11011011 = 8 + 4 + 0 + 1 = 13 y 8 + 0 + 2 + 1 = 11
How do we represent thirteen and eleven with only one digit each? In hexadecimal, values from ten to fifteen are represented by letters.
| Decimal | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
| Hexadecimal | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |
In the example above, the hexadecimal value of 11011011 is DB.
Practising hexadecimal/binary conversion
A good way to learn is by doing, and this is what I propose to do below. Let’s look at the definition of all the UDGs we are going to use (the hexadecimal values) and draw them by doing the conversion from hexadecimal to binary.
If we work with nibbles (4 bits), the conversion table for a byte would be as follows:
| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
We will create a folder called Step01 and inside it a file called var.asm. Having this table at hand, we are going to draw the ship, whose definition in hexadecimal (which we are going to copy into the created file) is the following:
udgsCommon:
db $24, $42, $99, $bd, $ff, $18, $24, $5a ; $90 ShipWe convert to binary.
| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $24 | X | X | ||||||
| $42 | X | X | ||||||
| $99 | X | X | X | X | ||||
| $bd | X | X | X | X | X | X | ||
| $ff | X | X | X | X | X | X | X | X |
| $18 | X | X | ||||||
| $24 | X | X | ||||||
| $5a | X | X | X | X |
If you transfer this conversion to ZX Paintbrush, or to the templates, the result should look like this:
We are going to continue practising, doing the conversions for the shot and the animation of the explosion of the ship. It is important that you try to translate it from hexadecimal to binary and from there to the graphics.
db $00, $18, $24, $5a, $5a, $24, $18, $00 ; $91 Shot| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $00 | ||||||||
| $18 | X | X | ||||||
| $24 | X | X | ||||||
| $5a | X | X | X | X | ||||
| $5a | X | X | X | X | ||||
| $24 | X | X | ||||||
| $18 | X | X | ||||||
| $00 |
db $00, $00, $00, $00, $24, $5a, $24, $18 ; $92 Blast 1| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $00 | ||||||||
| $00 | ||||||||
| $00 | ||||||||
| $00 | ||||||||
| $24 | X | X | ||||||
| $5a | X | X | X | X | ||||
| $24 | X | X | ||||||
| $18 | X | X |
db $00, $00, $00, $14, $2a, $34, $24, $18 ; $93 Blast 2| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $00 | ||||||||
| $00 | ||||||||
| $00 | ||||||||
| $14 | X | X | ||||||
| $2a | X | X | X | |||||
| $34 | X | X | X | |||||
| $24 | X | X | ||||||
| $18 | X | X |
db $00, $00, $0c, $12, $2a, $56, $64, $18 ; $94 Blast 3| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $00 | ||||||||
| $00 | ||||||||
| $0c | X | X | ||||||
| $12 | X | X | ||||||
| $2a | X | X | X | |||||
| $56 | X | X | X | X | ||||
| $64 | X | X | X | |||||
| $18 | X | X |
db $20, $51, $92, $d5, $a9, $72, $2c, $18 ; $95 Blast 4| Byte | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Value | 8 | 4 | 2 | 1 | 8 | 4 | 2 | 1 |
| $20 | X | |||||||
| $51 | X | X | X | |||||
| $92 | X | X | X | |||||
| $d5 | X | X | X | X | X | |||
| $a9 | X | X | X | X | ||||
| $72 | X | X | X | X | ||||
| $2c | X | X | X | |||||
| $18 | X | X |
From here on, I will just give the hexadecimal definition and the image of the final appearance of each UDG.
If you are wondering what the number in the comments of each definition means, it is the code of the character we are redefining, i.e. the character code we are going to send to the printer to make the graphic. Don’t worry if you don’t understand it now, it will be much clearer later.
db $3f, $6a, $ff, $b8, $f3, $a7, $ef, $ae ; $96 Top/Left
db $ff, $aa, $ff, $00, $ff, $ff, $00, $00 ; $97 Top
db $fc, $ae, $fb, $1f, $cd, $e7, $f5, $77 ; $98 Top/Right
db $ec, $ac, $ec, $ac, $ec, $ac, $ec, $ac ; $99 Left
db $35, $37, $35, $37, $35, $37, $35, $37 ; $9a Right
db $ee, $af, $e7, $b3, $f8, $df, $75, $3f ; $9b Bottom/Left
db $00, $00, $ff, $ff, $00, $ff, $55, $ff ; $9c Bottom
db $75, $f7, $e5, $cf, $1d, $ff, $56, $fc ; $9d Bottom/Right
db $00, $00, $00, $00, $00, $00, $00, $00 ; $9e White
udgsEnemiesLevel1:
db $8c, $42, $2d, $1d, $b4, $be, $46, $30 ; $9f Left/Up
db $31, $42, $b4, $b8, $2d, $7d, $62, $0c ; $a0 Right/Up
db $30, $46, $be, $b4, $1d, $2d, $42, $8c ; $a1 Left/Down
db $0c, $62, $7d, $2d, $b8, $b4, $42, $31 ; $a2 Right/Down
udgsEnemiesLevel2:
db $c0, $fb, $69, $5d, $7b, $14, $4a, $79 ; $9f Left/Up
db $03, $df, $96, $ba, $de, $28, $52, $9e ; $a0 Right/Up
db $79, $4a, $14, $7b, $5d, $69, $fb, $c0 ; $a1 Left/Down
db $9e, $52, $28, $de, $ba, $96, $df, $03 ; $a2 Right/Down
udgsEnemiesLevel3:
db $fc, $84, $b4, $af, $99, $f7, $14, $1c ; $9f Left/Up
db $3f, $21, $2d, $f5, $99, $ef, $28, $38 ; $a0 Right/Up
db $1c, $14, $f7, $99, $af, $b4, $84, $fc ; $a1 Left/Down
db $38, $28, $ef, $99, $f5, $2d, $21, $3f ; $a2 Right/Down
udgsEnemiesLevel4:
db $f2, $95, $98, $fe, $39, $55, $92, $4d ; $9f Left/Up
db $4f, $a9, $19, $7f, $9c, $aa, $49, $b2 ; $a0 Right/Up
db $4d, $92, $55, $39, $fe, $98, $95, $f2 ; $a1 Left/Down
db $b2, $49, $aa, $9c, $7f, $19, $a9, $4f ; $a2 Right/Down
udgsEnemiesLevel5:
db $76, $99, $a4, $d4, $47, $bd, $8a, $4c ; $9f Left/Up
db $6e, $99, $25, $2b, $e2, $bd, $51, $32 ; $a0 Right/Up
db $4c, $8a, $bd, $47, $d4, $a4, $99, $76 ; $a1 Left/Down
db $32, $51, $bd, $e2, $2b, $25, $99, $6e ; $a2 Right/Down
udgsEnemiesLevel6:
db $98, $66, $59, $aa, $b6, $49, $5a, $24 ; $9f Left/Up
db $19, $66, $9a, $55, $6d, $92, $5a, $24 ; $a0 Right/Up
db $24, $5a, $49, $b6, $aa, $59, $66, $98 ; $a1 Left/Down
db $24, $5a, $92, $6d, $55, $9a, $66, $19 ; $a2 Right/Down
udgsEnemiesLevel7:
db $04, $72, $5d, $74, $2e, $be, $4c, $20 ; $9f Left/Up
db $20, $4e, $ba, $2e, $74, $7d, $32, $04 ; $a0 Right/Up
db $20, $4c, $be, $2e, $74, $5d, $72, $04 ; $a1 Left/Down
db $04, $32, $7d, $74, $2e, $ba, $4e, $20 ; $a2 Right/Down
udgsEnemiesLevel8:
db $00, $7c, $5a, $68, $7c, $4f, $26, $04 ; $9f Left/Up
db $00, $3e, $5a, $16, $3e, $f2, $64, $20 ; $a0 Right/Up
db $04, $26, $4f, $7c, $68, $5a, $7c, $00 ; $a1 Left/Down
db $20, $64, $f2, $3e, $16, $5a, $3e, $00 ; $a2 Right/Down
udgsEnemiesLevel9:
db $e0, $d8, $b6, $6e, $5b, $36, $3c, $08 ; $9f Left/Up
db $07, $1b, $6d, $76, $da, $6c, $3c, $10 ; $a0 Right/Up
db $08, $3c, $36, $5b, $6e, $b6, $d8, $e0 ; $a1 Left/Down
db $10, $3c, $6c, $da, $76, $6d, $1b, $07 ; $a2 Right/Down
udgsEnemiesLevel10:
db $e0, $ce, $bf, $3c, $73, $75, $6a, $2c ; $9f Left/Up
db $07, $73, $fd, $3c, $ce, $ae, $56, $34 ; $a0 Right/Up
db $2c, $6a, $75, $73, $3c, $bf, $ce, $e0 ; $a1 Left/Down
db $34, $56, $ae, $ce, $3c, $fd, $73, $07 ; $a2 Right/Down
udgsEnemiesLevel11:
db $e0, $de, $bf, $7c, $7b, $75, $6a, $2c ; $9f Left/Up
db $07, $7b, $fd, $3e, $de, $ae, $56, $34 ; $a0 Right/Up
db $2c, $6a, $75, $7b, $7c, $bf, $de, $e0 ; $a1 Left/Down
db $34, $56, $ae, $de, $3e, $fd, $7b, $07 ; $a2 Right/Down
udgsEnemiesLevel12:
db $e0, $fe, $f7, $6c, $5f, $7e, $6c, $28 ; $9f Left/Up
db $07, $7f, $ef, $36, $fa, $7e, $36, $14 ; $a0 Right/Up
db $28, $6c, $7e, $5f, $6c, $f7, $fe, $e0 ; $a1 Left/Down
db $14, $36, $7e, $fa, $36, $ef, $7f, $07 ; $a2 Right/Down
udgsEnemiesLevel13:
db $07, $6c, $7e, $34, $6f, $fb, $ae, $8c ; $9f Left/Up
db $e0, $36, $7e, $2c, $f6, $df, $75, $31 ; $a0 Right/Up
db $8c, $ae, $fb, $6f, $34, $7e, $6c, $07 ; $a1 Left/Down
db $31, $75, $df, $f6, $2c, $7e, $36, $e0 ; $a2 Right/Down
udgsEnemiesLevel14:
db $21, $1a, $96, $75, $4c, $3c, $62, $90 ; $9f Left/Up
db $84, $58, $69, $ae, $32, $3c, $46, $09 ; $a0 Right/Up
db $90, $62, $3c, $4c, $75, $96, $1a, $21 ; $a1 Left/Down
db $09, $46, $3c, $32, $ae, $69, $58, $84 ; $a2 Right/Down
udgsEnemiesLevel15:
db $04, $02, $0d, $14, $28, $b0, $40, $20 ; $9f Left/Up
db $20, $40, $b0, $28, $14, $0d, $02, $04 ; $a0 Right/Up
db $20, $40, $b0, $28, $14, $0d, $02, $04 ; $a1 Left/Down
db $04, $02, $0d, $14, $28, $b0, $40, $20 ; $a2 Right/Down
udgsEnemiesLevel16:
db $30, $48, $be, $b9, $7c, $2e, $27, $13 ; $9f Left/Up
db $0c, $12, $7d, $9d, $3e, $74, $e4, $c8 ; $a0 Right/Up
db $13, $27, $2e, $7c, $b9, $be, $48, $30 ; $a1 Left/Down
db $c8, $e4, $74, $3e, $9d, $7d, $12, $0c ; $a2 Right/Down
udgsEnemiesLevel17:
db $c0, $df, $36, $7c, $58, $77, $66, $44 ; $9f Left/Up
db $03, $fb, $6c, $3e, $1a, $ee, $66, $22 ; $a0 Right/Up
db $44, $66, $77, $58, $7c, $36, $df, $c0 ; $a1 Left/Down
db $22, $66, $ee, $1a, $3e, $6c, $fb, $03 ; $a2 Right/Down
udgsEnemiesLevel18:
db $02, $71, $69, $57, $2f, $1e, $9e, $78 ; $9f Left/Up
db $40, $8e, $96, $ea, $f4, $78, $79, $1e ; $a0 Right/Up
db $78, $9e, $1e, $2f, $57, $69, $71, $02 ; $a1 Left/Down
db $1e, $79, $78, $f4, $ea, $96, $8e, $40 ; $a2 Right/Down
udgsEnemiesLevel19:
db $20, $7f, $e6, $4e, $5e, $79, $78, $44 ; $9f Left/Up
db $04, $fe, $67, $72, $7a, $9e, $1e, $22 ; $a0 Right/Up
db $44, $78, $79, $5e, $4e, $e6, $7f, $20 ; $a1 Left/Down
db $22, $1e, $9e, $7a, $72, $67, $fe, $02 ; $a2 Right/Down
udgsEnemiesLevel20:
db $36, $2f, $db, $be, $7c, $db, $f6, $64 ; $9f Left/Up
db $6c, $f4, $db, $7d, $3e, $db, $6f, $26 ; $a0 Right/Up
db $64, $f6, $db, $7c, $be, $db, $2f, $36 ; $a1 Left/Down
db $26, $6f, $db, $3e, $7d, $db, $f4, $6c ; $a2 Right/Down
udgsEnemiesLevel21:
db $00, $70, $6e, $54, $2b, $34, $28, $08 ; $9f Left/Up
db $00, $0e, $76, $2a, $d4, $2c, $14, $10 ; $a0 Right/Up
db $08, $28, $34, $2b, $54, $6e, $70, $00 ; $a1 Left/Down
db $10, $14, $2c, $d4, $2a, $76, $0e, $00 ; $a2 Right/Down
udgsEnemiesLevel22:
db $00, $78, $6e, $56, $6d, $3b, $34, $0c ; $9f Left/Up
db $00, $1e, $76, $6a, $b6, $dc, $2c, $30 ; $a0 Right/Up
db $0c, $34, $3b, $6d, $56, $6e, $78, $00 ; $a1 Left/Down
db $30, $2c, $dc, $b6, $6a, $76, $1e, $00 ; $a2 Right/Down
udgsEnemiesLevel23:
db $0c, $02, $3d, $35, $ac, $b8, $40, $30 ; $9f Left/Up
db $30, $40, $bc, $ac, $35, $1d, $02, $0c ; $a0 Right/Up
db $30, $40, $b8, $ac, $35, $3d, $02, $0c ; $a1 Left/Down
db $0c, $02, $1d, $35, $ac, $bc, $40, $30 ; $a2 Right/Down
udgsEnemiesLevel24:
db $00, $77, $6e, $56, $2a, $74, $7b, $42 ; $9f Left/Up
db $00, $ee, $76, $6a, $54, $2e, $de, $42 ; $a0 Right/Up
db $42, $7b, $74, $2a, $56, $6e, $77, $00 ; $a1 Left/Down
db $42, $de, $2e, $54, $6a, $76, $ee, $00 ; $a2 Right/Down
udgsEnemiesLevel25:
db $c0, $ff, $76, $6c, $5f, $7e, $6c, $48 ; $9f Left/Up
db $03, $ff, $6e, $36, $fa, $7e, $36, $12 ; $a0 Right/Up
db $48, $6c, $7e, $5f, $6c, $76, $ff, $c0 ; $a1 Left/Down
db $12, $36, $7e, $fa, $36, $6e, $ff, $03 ; $a2 Right/Down
udgsEnemiesLevel26:
db $3c, $7e, $f7, $e8, $da, $e1, $68, $24 ; $9f Left/Up
db $3c, $7e, $ef, $17, $5b, $87, $16, $24 ; $a0 Right/Up
db $24, $68, $e1, $da, $e8, $f7, $7e, $3c ; $a1 Left/Down
db $24, $16, $87, $5b, $17, $ef, $7e, $3c ; $a2 Right/Down
udgsEnemiesLevel27:
db $04, $02, $39, $2d, $3f, $9e, $4c, $38 ; $9f Left/Up
db $20, $40, $9c, $b4, $fc, $79, $32, $1c ; $a0 Right/Up
db $38, $4c, $9e, $3f, $2d, $39, $02, $04 ; $a1 Left/Down
db $1c, $32, $79, $fc, $b4, $9c, $40, $20 ; $a2 Right/Down
udgsEnemiesLevel28:
db $00, $37, $69, $5c, $34, $5f, $46, $64 ; $9f Left/Up
db $00, $ec, $96, $3a, $2c, $fa, $62, $26 ; $a0 Right/Up
db $64, $46, $5f, $34, $5c, $69, $37, $00 ; $a1 Left/Down
db $26, $62, $fa, $2c, $3a, $96, $ec, $00 ; $a2 Right/Down
udgsEnemiesLevel29:
db $00, $37, $6d, $5e, $34, $7f, $56, $64 ; $9f Left/Up
db $00, $ec, $b6, $7a, $2c, $fe, $6a, $26 ; $a0 Right/Up
db $64, $56, $7f, $34, $5e, $6d, $37, $00 ; $a1 Left/Down
db $26, $6a, $fe, $2c, $7a, $b6, $ec, $00 ; $a2 Right/Down
udgsEnemiesLevel30:
db $e0, $ff, $ed, $5b, $7e, $6e, $5f, $72 ; $9f Left/Up
db $07, $ff, $b7, $da, $7e, $76, $fa, $4e ; $a0 Right/Up
db $72, $5f, $6e, $7e, $5b, $ed, $ff, $e0 ; $a1 Left/Down
db $4e, $fa, $76, $7e, $da, $b7, $ff, $07 ; $a2 Right/Down
With this we have already defined the graphics we are going to use: the ship, the shot, the explosion, the frame of the screen and the enemies.
You may notice that all the enemies have the same character code, this is because there is a limited number of characters that can be used as UDGs. Don’t worry, we’ll see a way around this later on.
ZX-Spectrum Assembly, Space Battle
It’s really very important to practice the hexadecimal to binary conversion, so don’t leave it for another day, it’s a very simple exercise.
In the next chapter of ZX Spectrum Assembly, we will start drawing with UDG.
Download the source code from here.
Useful links
ZX Spectrum Assembly, Space Battle by Juan Antonio Rubio García.
Translation by Felipe Monge Corbalán.
This work is licensed to Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0).
Any comments are always welcome.