; ; clearmem.asm ; ; Blitter example---memory clear (link with amiga.lib) ; include 'exec/types.i' include 'hardware/custom.i' include 'hardware/dmabits.i' include 'hardware/blit.i' include 'hardware/hw_examples.i" xref _custom ; ; Wait for previous blit to complete. ; waitblit: btst.b #DMAB_BLTDONE-8,DMACONR(a1) waitblit2: btst.b #DMAB_BLTDONE-8,DMACONR(a1) bne waitblit2 rts ; ; This routine uses a side effect in the blitter. When each ; of the blits is finished, the pointer in the blitter is pointing ; to the next word to be blitted. ; ; When this routine returns, the last blit is started and might ; not be finished, so be sure to call waitblit above before ; assuming the data is clear. ; ; a0 = pointer to first word to clear ; d0 = number of bytes to clear (must be even) ; xdef clearmem clearmem: lea _custom,a1 ; Get pointer to chip registers bsr waitblit ; Make sure previous blit is done move.l a0,BLTDPT(a1) ; Set up the D pointer to the region to clear clr.w BLTDMOD(a1) ; Clear the D modulo (don't skip no bytes) asr.l #1,d0 ; Get number of words from number of bytes clr.w BLTCON1(a1) ; No special modes move.w #DEST,BLTCON0(a1) ; only enable destination ; ; First we deal with the smaller blits ; moveq #$3f,d1 ; Mask out mod 64 words and.w d0,d1 beq dorest ; none? good, do one blit sub.l d1,d0 ; otherwise remove remainder or.l #$40,d1 ; set the height to 1, width to n move.w d1,BLTSIZE(a1) ; trigger the blit ; ; Here we do the rest of the words, as chunks of 128k ; dorest: move.w #$ffc0,d1 ; look at some more upper bits and.w d0,d1 ; extract 10 more bits beq dorest2 ; any to do? sub.l d1,d0 ; pull of the ones we're doing here bsr waitblit ; wait for prev blit to complete move.w d0,BLTSIZE(a1) ; do another blit dorest2: swap d0 ; more? beq done ; nope. clr.w d1 ; do a 1024x64 word blit (128K) keepon: bsr waitblit ; finish up this blit move.w d1,BLTSIZE(a1) ; and again, blit subq.w #1,d0 ; still more? bne keepon ; keep on going. done: rts ; finished. Blit still in progress. end
: Roy Schneider says (in March 2019):
"I found a bug in the Blitter based memory clear function [...]
This must be 'd1'. Othwerwise you overwrite memory."
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