ZX Spectrum Screen Memory Layout

The Spectrum screen memory map is split into two sections:

6144 bytes worth of bitmap data, starting at memory address &4000 (16384d)
768 byte colour attribute data, immediately after the bitmap data at address &5800 (22528d)

Bitmap data layout

The bitmap data starts at address &4000 and consists of 192 lines of 32 bytes. Pixels are encoded as bits; 32 x 8 gives the horizontal resolution of 256 pixels. This example is a layout of the top line of the display with the pixels set at X position 0 and 255.

The display lines are not placed linearly; that is, each line is not 32 bytes beneath the one above it. To calculate the screen address of a byte, you encode the address as follows:

Where:

The base address of screen memory (&4000) is provided by setting bits 15 to 13 to 010.
Y0 to Y7 is the Y coordinate (in pixels)
X0 to X4 is the X coordinate (in bytes)

Note the bit positions of Y0-Y2 and Y3-Y5 are transposed. The advantages of this is that you can quickly get to the next line within a character boundary by increasing the high byte of the address. The disadvantage is that the code to calculate the screen position or get the next line past a character boundary is quite convoluted:

; Get screen address
; B = Y pixel position
; C = X pixel position
; Returns address in HL

Calculate_Pixel_Address:
        LD A,B                                 ; Calculate Y2,Y1,Y0
        AND %00000111                          ; Mask out unwanted bits
        OR %01000000                           ; Set base address of screen
        LD H,A                                 ; Store in H
        LD A,B                                 ; Calculate Y7,Y6
        RRA                                    ; Shift to position
        RRA
        RRA
        AND %00011000                          ; Mask out unwanted bits
        OR H                                   ; OR with Y2,Y1,Y0
        LD H,A                                 ; Store in H
        LD A,B                                 ; Calculate Y5,Y4,Y3
        RLA                                    ; Shift to position
        RLA
        AND %11100000                          ;Mask out unwanted bits
        LD L,A                                 ; Store in L
        LD A,C                                 ; Calculate X4,X3,X2,X1,X0
        RRA                                    ; Shift into position
        RRA
        RRA
        AND %00011111                          ; Mask out unwanted bits
        OR L                                   ; OR with Y5,Y4,Y3
        LD L,A                                 ; Store in L
        RET

Colour attribrutes data layout

The colour attribute data overlays the monochrome bitmap data and is arranged in a linear fashion from left to right, top to bottom.

Each attribute byte colours an 8×8 character on the screen and is encoded as follows:

Where:

F sets the attribute FLASH mode
B sets the attribute BRIGHTNESS mode
P2 to P0 is the PAPER colour
I2 to I0 is the INK colour

So for each 8×8 character position you can only set two colours, this limitation is the sole cause of what is affectionately known as “attribute clash”.