Os caracteres que se podem definir vão de $90 até $A4 ; cada um define-se num block de 8x8. A rotina da ROM usa esses bitmaps a partir do endereço que estiver definido na variável UDG $5C7B.
Como exemplo, definimos UDGs com as componentes de um rectângulo, e imprimimos usando rotinas da ROM:
LD HL,UDG
LD ($5C7B),HL ; points UDG system var pointer to UDG here
; print string STR
; ended by 0
LD HL,STR
PRINT: LD A,(HL)
CP 0
RET Z
RST $10
INC HL
JR PRINT
; string to print
; each line is a DEFB line, set of UDG characters, or space ($20)
; ended by newline ($0D)
; 0 to end the string
STR:
DEFB $90, $91, $91, $91, $92, $0D
DEFB $93, $20, $20, $20, $94, $0D
DEFB $93, $20, $20, $20, $94, $0D
DEFB $93, $20, $20, $20, $94, $0D
DEFB $93, $20, $20, $20, $94, $0D
DEFB $95, $96, $96, $96, $97, 0
; UDG area
UDG:
; $90
DEFB %11111111
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
; $91
DEFB %11111111
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
; $92
DEFB %11111111
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
; $93
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
; $94
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
; $95
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %10000000
DEFB %11111111
; $96
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %00000000
DEFB %11111111
; $97
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %00000001
DEFB %11111111
END 50000
Ver:
ZX Spectrum - modo "texto" (I)
ZX Basic Manual - The character set
The Moon (UDG Moon’s Phases)
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