Clear the Screen
Example code to clear the Spectrum’s screen
This code snippet will clear the standard screen on all versions of the Spectrum.
It's broken down into three sections:
- Clear's the pixel data
- Reset the screen's colour data, in this instance to White text on Black Background
- Sets the screen border to black
; ***************************************************************************
; Clear the Spectrum Screen
;
; Example 1
;
; Author: Peter Mount, Area51.dev & Contributors
; URL: https://area51.dev/sinclair/asm/screen/getcharaddr/
; ***************************************************************************
;
; Clear the Spectrum screen, setting the border to black and white text on
; a black background
;
; On Exit:
; A Undefined
; BC Undefined
; DE Undefined
; HL Undefined
;
clearScreen:
; Clear the pixel data
ld hl,DISPLAYFILE ; Start of screen memory
ld (hl),&00 ; Set first byte to 0
ld de,DISPLAYFILE+1 ; Destination as next byte
ld bc, 6144 ; Number of bytes to copy
ldir ; Copy from hl to de bc times
; Clear the colour attributes
ld (hl),&07 ; White text black Background
ld bc, 767 ; Number of bytes to copy, attr size -1
ldir ; Clear the attributes
ld a, 0 ; Set border to black
ld (BORDCR),a ; set OS copy of colour
out (&fe),a ; set hardware colour
ret
Important note here:
We copy 6144 bytes in the first LDIR not 6143 which we would normally do if we want to erase just the
DISPLAYFILE.
Using 6144 bytes will cause the first byte of DISPLAYATTR to be set to 0 which is fine here as we want
HL and DE to be pointing to DISPLAYATTR ready for the second
LDIR operation.
This could, in theory cause the top left character to flicker briefly as it's been set to 0
(BLACK foreground and background).
If that's an issue then set the BC to 6143 and increment HL and DE after the
LDIR as in the next example:
; ***************************************************************************
; Clear the Spectrum Screen
;
; Example 2 - does not corrupt the top left characters colour attributes
;
; Author: Peter Mount, Area51.dev & Contributors
; URL: https://area51.dev/sinclair/asm/screen/getcharaddr/
; ***************************************************************************
;
; Clear the Spectrum screen, setting the border to black and white text on
; a black background
;
; On Exit:
; A Undefined
; BC Undefined
; DE Undefined
; HL Undefined
;
clearScreen:
; Clear the pixel data
ld hl,DISPLAYFILE ; Start of screen memory
ld (hl),&00 ; Set first byte to 0
ld de,DISPLAYFILE+1 ; Destination as next byte
ld bc, 6143 ; Number of bytes to copy
ldir ; Copy from hl to de bc times
inc hl ; Move HL and DE forward 1 byte
inc de ; so they point to DISPLAYATTR
; Clear the colour attributes
ld (hl),&07 ; White text black Background
ld bc, 767 ; Number of bytes to copy, attr size -1
ldir ; Clear the attributes
ld a, 0 ; Set border to black
ld (BORDCR),a ; set OS copy of colour
out (&fe),a ; set hardware colour
ret
There are faster but longer methods of clearing the screen which I won't go into here.
For most purposes the first example above will work for most purposes other than time critical games.