Z80 Routines:Graphic:Fastcopy

The Fastcopy routine is used to copy the content of the Graph Buffer to the screen. It concerns all TI-z80 calculators, except TI-85 and TI-86, that has a special RAM area directly mapped to the screen.

Fastcopy is widely used, because the rom call _GrBufCpy waits too long between each output to the LCD driver. Using Fastcopy instead of _GrBufCpy increases significantly the speed of a program if it refreshes often the display (such as a lot of games). Most shells have this routine as built-in (ION, MirageOS, Venus ...).

Here is Joe Wingbermuehle's version, which is the one used in ION.

;-----> Copy the gbuf to the screen (fast)
;Input: nothing
;Output:graph buffer is copied to the screen
fastCopy:
 di
 ld a,$80
 out ($10),a
 ld hl,gbuf-12-(-(12*64)+1)
 ld a,$20
 ld c,a
 inc hl
 dec hl
fastCopyAgain:
 ld b,64
 inc c
 ld de,-(12*64)+1
 out ($10),a
 add hl,de
 ld de,10
fastCopyLoop:
 add hl,de
 inc hl
 inc hl
 inc de
 ld a,(hl)
 out ($11),a
 dec de
 djnz fastCopyLoop
 ld a,c
 cp $2B+1
 jr nz,fastCopyAgain
 ret

Remarks and Improvements

• The di instruction is dispensable.

• Some instructions in Joe Wingbermuehle's Fastcopy are only there for having enough delay between two outputs to the LCD driver. One can modify Fastcopy to mange these useless instruction into instructions that will cler the Graph Buffer at the same time :

;-----> Copy the gbuf to the screen (fast)
;Input: nothing
;Output:graph buffer is copied to the screen
fastCopy:
 di
 ld a,$80
 out ($10),a
 ld hl,gbuf-12-(-(12*64)+1)
 ld a,$20
 ld c,a
 inc hl
 dec hl
fastCopyAgain:
 ld b,64
 inc c
 ld de,-(12*64)+1
 out ($10),a
 add hl,de
 ld de,11
fastCopyLoop:
 add hl,de
 inc hl
 inc de
 ld a,(hl)
 ld (hl),0    ; clears the graph buffer at the same time
 out ($11),a
 dec de
 djnz fastCopyLoop
 ld a,c
 cp $2B+1
 jr nz,fastCopyAgain
 ret

ld (hl),0 takes 10 cycles, so we can do with one less inc hl (6 cycles) and thus load 11 into de instead of 10.