ZX Spectrum Assembly, Space Battle – 0x01 Definition of graphics, Espamatica

17/09/2023

Espamatica : ZX Spectrum Assembly, Space Battle – 0x01 Definition of graphics Leer

In this chapter of ZX Spectrum Assembly, we will define all the graphics used in Space Battle.

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

ZX Spectrum Assembly, Space Battle

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

ZX Spectrum Assembly, Space Battle

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

ZX Spectrum Assembly, Space Battle

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 Ship

We 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

ZX Spectrum Assembly, Space Battle

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