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Magnus Holmgrens improved word swap

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2529 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Linus Nielsen Feltzing 2002-10-08 12:14:13 +00:00
parent 1e972b53e5
commit 095c68bc1d
1 changed files with 44 additions and 16 deletions
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60
firmware/bitswap.S
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@ -25,28 +25,56 @@
/* Registers used: /* Registers used:
* *
* r0 Temporary (required by some instructions) * r0 Temporary (required by some instructions)
* r1 Flip table * r1 Low byte
* r4 Argument: Data pointer * r2 High byte
* r5 Argument: Length (in bytes) * r3 Result after flip
* r4 Data
* r5 Length
* r6 1
* r7 Flip table
*/ */
/* TODO: Optimize for DRAM burst operation by reading and writing in chunks
We do not want to optimize by reading/writing words, because the data
pointer may be odd */
_bitswap: _bitswap:
mov.l .fliptable,r1 mov.l .fliptable,r7
add r4,r5 /* Calculate end of buffer */ mov #1,r6
mov r4,r0
and #1,r0 /* odd address? */
cmp/eq #0,r0
bt .init /* no, address is even */
mov.b @r4,r0 /* swap first byte */
extu.b r0,r0
mov.b @(r0,r7),r0
mov.b r0,@r4
add #1,r4
add #-1,r5
bra .init bra .init
.loop: .loop:
mov.b @r4,r0 /* Data to flip */ mov.w @r4,r1 /* data to flip */
extu.b r0,r0 /* Zero extend */ swap.b r1,r2
mov.b @(r0,r1),r0 /* Look up in the flip table */ extu.b r2,r0 /* high byte */
mov.b r0,@r4 /* Store result */ mov.b @(r0,r7),r2
add #1,r4 extu.b r2,r0 /* Zero extend */
swap.b r0,r3 /* put high byte in result */
extu.b r1,r0 /* low byte */
mov.b @(r0,r7),r1
extu.b r1,r0 /* Zero extend */
or r0,r3 /* put low byte in result */
mov.w r3,@r4 /* store result */
add #2,r4
add #-2,r5
.init: .init:
cmp/gt r4,r5 /* while (dataptr < start+length) */ cmp/gt r6,r5 /* while [bytes remaining] > 1 */
bt .loop bt .loop /* (at least 2 bytes left) */
cmp/eq r6,r5
bf .exit /* if not 1 byte left, exit */
mov.b @r4,r0 /* swap last byte */
extu.b r0,r0
mov.b @(r0,r7),r0
mov.b r0,@r4
.exit:
rts rts
.align 4 .align 4