mirror of
https://github.com/eried/portapack-mayhem.git
synced 2024-10-01 01:26:06 -04:00
70 lines
4.9 KiB
ArmAsm
70 lines
4.9 KiB
ArmAsm
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/* source: https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/lz4-decompression-routine-for-cortex-m0-and-later */
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.syntax unified
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.cpu cortex-m0
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.thumb
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/* License: Public Domain - I cannot be held responsible for what it does or does not do if you use it, whether it's modified or not. */
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/* Entry point = unlz4. On entry: r0 = source, r1 = destination. The first two bytes of the source must contain the length of the compressed data. */
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.func unlz4
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.global unlz4,unlz4_len
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.type unlz4,%function
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.type unlz4_len,%function
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.thumb_func
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unlz4: ldrh r2,[r0] /* get length of compressed data */
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adds r0,r0,#2 /* advance source pointer */
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.thumb_func
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unlz4_len: push {r4-r6,lr} /* save r4, r5, r6 and return-address */
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adds r5,r2,r0 /* point r5 to end of compressed data */
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getToken: ldrb r6,[r0] /* get token */
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adds r0,r0,#1 /* advance source pointer */
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lsrs r4,r6,#4 /* get literal length, keep token in r6 */
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beq getOffset /* jump forward if there are no literals */
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bl getLength /* get length of literals */
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movs r2,r0 /* point r2 to literals */
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bl copyData /* copy literals (r2=src, r1=dst, r4=len) */
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movs r0,r2 /* update source pointer */
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getOffset: ldrb r3,[r0,#0] /* get match offset's low byte */
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subs r2,r1,r3 /* subtract from destination; this will become the match position */
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ldrb r3,[r0,#1] /* get match offset's high byte */
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lsls r3,r3,#8 /* shift to high byte */
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subs r2,r2,r3 /* subtract from match position */
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adds r0,r0,#2 /* advance source pointer */
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lsls r4,r6,#28 /* get rid of token's high 28 bits */
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lsrs r4,r4,#28 /* move the 4 low bits back where they were */
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bl getLength /* get length of match data */
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adds r4,r4,#4 /* minimum match length is 4 bytes */
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bl copyData /* copy match data (r2=src, r1=dst, r4=len) */
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cmp r0,r5 /* check if we've reached the end of the compressed data */
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blt getToken /* if not, go get the next token */
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pop {r4-r6,pc} /* restore r4, r5 and r6, then return */
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.thumb_func
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getLength: cmp r4,#0x0f /* if length is 15, then more length info follows */
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bne gotLength /* jump forward if we have the complete length */
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getLengthLoop: ldrb r3,[r0] /* read another byte */
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adds r0,r0,#1 /* advance source pointer */
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adds r4,r4,r3 /* add byte to length */
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cmp r3,#0xff /* check if end reached */
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beq getLengthLoop /* if not, go round loop */
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gotLength: bx lr /* return */
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.thumb_func
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copyData: rsbs r4,r4,#0 /* index = -length */
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subs r2,r2,r4 /* point to end of source */
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subs r1,r1,r4 /* point to end of destination */
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copyDataLoop: ldrb r3,[r2,r4] /* read byte from source_end[-index] */
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strb r3,[r1,r4] /* store byte in destination_end[-index] */
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adds r4,r4,#1 /* increment index */
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bne copyDataLoop /* keep going until index wraps to 0 */
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bx lr /* return */
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.size unlz4,.-unlz4
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.endfunc
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/* 42 narrow instructions = 84 bytes */
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