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114 lines
3.9 KiB
ArmAsm
114 lines
3.9 KiB
ArmAsm
;
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; Math constants for scaling.
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;
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; Each table has 16 sets of 16 entries, with one set for each of the 16 possible
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; scale values. The values within a set determine how one 4-bit nibble of the
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; coordinate is scaled.
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;
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; Suppose you want to scale the value 100 ($64) by scale factor 8 (a bit over
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; half size). We begin by using self-modifying code to select the table
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; subsets. This is done in a clever way to avoid shifting. The instructions
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; that load from ScaleTabLo are modified to reference $1800, $1810, $1820, and
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; so on. The instructions that load from ScaleTabHi reference $1900, $1901,
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; $1902, etc. The offset comes from the two 16-byte ScaleIndex tables. For a
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; scale factor of 8, we'll be using $1880 and $1908 as the base addresses.
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;
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; To do the actual scaling, we mask to get the low part of the value ($04) and
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; index into ScaleTabLo, at address $1884. We mask the high part of the value
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; ($60) and index into ScaleTabHi, at $1968. We add the values there ($02, $36)
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; to get $38 = 56, which is just over half size as expected.
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;
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; This is an approximation, but so is any integer division, and it's done in
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; 512+32=544 bytes instead of the 16*256=4096 bytes that you'd need for a fully-
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; formed scale. For hi-res graphics it's certainly good enough.
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;
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; 32 = $20 = ($1880)+($1928) = 18 (.563)
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; 40 = $28 = ($1888)+($1928) = 22 (.55)
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; 47 = $2F = ($188F)+($1928) = 26 (.553)
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; 48 = $30 = ($1880)+($1938) = 27 (.563)
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; 100 = $64 = ($1884)+($1968) = 56 (.56)
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;
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ScaleTabLo:
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.byte $00,$00,$00,$00,$00,$00,$00,$00
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.byte $00,$00,$00,$00,$00,$00,$00,$00
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.byte $00,$00,$00,$00,$00,$00,$00,$00
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.byte $01,$01,$01,$01,$01,$01,$01,$01
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.byte $00,$00,$00,$00,$00,$00,$01,$01
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.byte $01,$01,$01,$02,$02,$02,$02,$02
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.byte $00,$00,$00,$00,$01,$01,$01,$01
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.byte $02,$02,$02,$02,$03,$03,$03,$03
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.byte $00,$00,$00,$00,$01,$01,$01,$02
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.byte $02,$02,$03,$03,$03,$04,$04,$04
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.byte $00,$00,$00,$01,$01,$01,$02,$02
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.byte $03,$03,$03,$04,$04,$04,$05,$05
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.byte $00,$00,$00,$01,$01,$02,$02,$03
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.byte $03,$03,$04,$04,$05,$05,$06,$06
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.byte $00,$00,$01,$01,$02,$02,$03,$03
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.byte $04,$04,$05,$05,$06,$06,$07,$07
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.byte $00,$00,$01,$01,$02,$02,$03,$03
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.byte $04,$05,$05,$06,$06,$07,$07,$08
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.byte $00,$00,$01,$01,$02,$03,$03,$04
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.byte $05,$05,$06,$06,$07,$08,$08,$09
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.byte $00,$00,$01,$02,$02,$03,$04,$04
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.byte $05,$06,$06,$07,$08,$08,$09,$0a
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.byte $00,$00,$01,$02,$03,$03,$04,$05
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.byte $06,$06,$07,$08,$09,$09,$0a,$0b
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.byte $00,$00,$01,$02,$03,$04,$04,$05
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.byte $06,$07,$08,$08,$09,$0a,$0b,$0c
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.byte $00,$00,$01,$02,$03,$04,$05,$06
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.byte $07,$07,$08,$09,$0a,$0b,$0c,$0d
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.byte $00,$00,$01,$02,$03,$04,$05,$06
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.byte $07,$08,$09,$0a,$0b,$0c,$0d,$0e
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.byte $00,$01,$02,$03,$04,$05,$06,$07
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.byte $08,$09,$0a,$0b,$0c,$0d,$0e,$0f
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ScaleTabHi:
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.byte $00,$00,$00,$00,$00,$00,$00,$00
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.byte $00,$00,$00,$00,$00,$00,$00,$00
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.byte $01,$02,$03,$04,$05,$06,$07,$08
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.byte $09,$0a,$0b,$0c,$0d,$0e,$0f,$10
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.byte $02,$04,$06,$08,$0a,$0c,$0e,$10
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.byte $12,$14,$16,$18,$1a,$1c,$1e,$20
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.byte $03,$06,$09,$0c,$0f,$12,$15,$18
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.byte $1b,$1e,$21,$24,$27,$2a,$2d,$30
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.byte $04,$08,$0c,$10,$14,$18,$1c,$20
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.byte $24,$28,$2c,$30,$34,$38,$3c,$40
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.byte $05,$0a,$0f,$14,$19,$1e,$23,$28
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.byte $2d,$32,$37,$3c,$41,$46,$4b,$50
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.byte $06,$0c,$12,$18,$1e,$24,$2a,$30
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.byte $36,$3c,$42,$48,$4e,$54,$5a,$60
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.byte $07,$0e,$15,$1c,$23,$2a,$31,$38
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.byte $3f,$46,$4d,$54,$5b,$62,$69,$70
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.byte $f8,$f0,$e8,$e0,$d8,$d0,$c8,$c0
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.byte $b8,$b0,$a8,$a0,$98,$90,$88,$80
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.byte $f9,$f2,$eb,$e4,$dd,$d6,$cf,$c8
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.byte $c1,$ba,$b3,$ac,$a5,$9e,$97,$90
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.byte $fa,$f4,$ee,$e8,$e2,$dc,$d6,$d0
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.byte $ca,$c4,$be,$b8,$b2,$ac,$a6,$a0
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.byte $fb,$f6,$f1,$ec,$e7,$e2,$dd,$d8
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.byte $d3,$ce,$c9,$c4,$bf,$ba,$b5,$b0
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.byte $fc,$f8,$f4,$f0,$ec,$e8,$e4,$e0
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.byte $dc,$d8,$d4,$d0,$cc,$c8,$c4,$c0
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.byte $fd,$fa,$f7,$f4,$f1,$ee,$eb,$e8
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.byte $e5,$e2,$df,$dc,$d9,$d6,$d3,$d0
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.byte $fe,$fc,$fa,$f8,$f6,$f4,$f2,$f0
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.byte $ee,$ec,$ea,$e8,$e6,$e4,$e2,$e0
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.byte $ff,$fe,$fd,$fc,$fb,$fa,$f9,$f8
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.byte $f7,$f6,$f5,$f4,$f3,$f2,$f1,$f0
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