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Screen Memory Layout

The Spectrum screen memory map is split into two sections:

  1. 6144 bytes worth of bitmap data, starting at memory address &4000
  2. 768 byte colour attribute data, immediately after the bitmap data at address &5800

Bitmap data

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
Get_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 attribute data

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”.