Display File

Address	Len	Name	Description
4000	6144	DISPLAYFILE	The start of the standard Spectrum's standard screen

The display file contains the pixel data for the screen.

Arranged in a map of 192 rows, each 32 bytes long. Each byte contains 8 pixels.

Be aware, the rows are not stored linearly, but in a way that has been optimised for the ULA.

The first 256 bytes in memory represents the first byte of each character in the first 8 rows, then the second byte of each character, and so on. Once the first 8 rows have been done it repeats twice more for the next 8 rows then the last 8.

Although the memory map for the spectrum screen seems weird with its layout, it's actually pretty logical. You can tell this weirdness is down to how the ULA works internally.

Calculating the characters address

If you have:

R with the screen row (0…23)
C with the column (0…31)
N for the byte offset within the character (0…7)

then you can calculate the address using the following table:

Screen Address
MSB								LSB
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	1	0	R₄	R₃	N₂	N₁	N₀	R₂	R₁	R₀	C₄	C₃	C₂	C₁	C₀

The formula is simple: $$ Address = \left( \overbrace{ ( 0x40 | ( R \And 0x18 ) | N ) }^{MSB} \ll 8 \right) | \left( \overbrace{ ( ( R \And 0x07) \ll 5 ) | C }^{LSB} \right) $$

Pseudo code:

1lsb := ((R << 5) AND 0xC0) OR C
2msb := 0x40 OR (R AND 0x18) OR N

Machine Code:

An example routine in Z80 Machine Code for this calculation is available to download.

Addresses of bytes within the same character

If you calculate the address of the top row of the character (i.e. N=0) then, when rendering the character to the screen you just need to increment the upper byte of the address.

This works as the lower 3 bits of the upper byte represent the row for the character.

The adjacent table shows the addresses for each row of the first 5 characters on row 0 of the screen.

Char Row	Address
Char Row	Char 0	Char 1	Char 2	Char 3	Char 4
0	4000	4001	4002	4003	4004
1	4100	4101	4102	4103	4104
2	4200	4201	4202	4203	4204
3	4300	4301	4302	4303	4304
4	4400	4401	4402	4403	4404
5	4500	4501	4502	4503	4504
6	4600	4601	4602	4603	4604
7	4700	4701	4702	4703	4704

Get address of the next character

You can get the address of the next character simply by adding one to the lower byte of the address. Be careful however when wrapping around the end of the line as, after the first 8 lines the address will fail.

Ideally you should calculate the address again at the start of each line.

This might explain why the original spectrum editor was 8 lines long, as you only needed to set the high byte to an appropriate 8-line block in memory and set the low byte to the offset in the line being edited to get that characters screen address.

A simple 256 character editor

As mentioned above, you could use one third of the screen as a simple editor which is capable of displaying 256 characters.

Address	MSB	Rows		Screen position
4000	0x40	0	7	Top third
4800	0x48	8	15	Middle third
5000	0x50	16	23	Bottom third

As long as you restrict yourself to one third of the screen you can simply calculate the address within this simple editor by selecting the high byte for the required section of the screen and use the characters position in the line as the low byte.