From 3ce9a2b36e9df25b3fd68fb27380f62ceb72e27e Mon Sep 17 00:00:00 2001
From: Emmanuel Marty <emmanuel@fgl.com>
Date: Thu, 18 Jun 2020 13:26:25 +0200
Subject: [PATCH] Add 6809 depacker for LZSA1

---
 asm/6809/unlzsa1.s | 109 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 asm/6809/unlzsa1.s

diff --git a/asm/6809/unlzsa1.s b/asm/6809/unlzsa1.s
new file mode 100644
index 0000000..b9c95af
--- /dev/null
+++ b/asm/6809/unlzsa1.s
@@ -0,0 +1,109 @@
+;  unlzsa1.s - 6809 decompression routine for raw LZSA1 - 125 bytes
+;  compress with lzsa -r <original_file> <compressed_file>
+;
+;  in:  x = start of compressed data
+;       y = start of decompression buffer
+;  out: y = end of decompression buffer + 1
+;
+;  Copyright (C) 2020 Emmanuel Marty
+;
+;  This software is provided 'as-is', without any express or implied
+;  warranty.  In no event will the authors be held liable for any damages
+;  arising from the use of this software.
+;
+;  Permission is granted to anyone to use this software for any purpose,
+;  including commercial applications, and to alter it and redistribute it
+;  freely, subject to the following restrictions:
+;
+;  1. The origin of this software must not be misrepresented; you must not
+;     claim that you wrote the original software. If you use this software
+;     in a product, an acknowledgment in the product documentation would be
+;     appreciated but is not required.
+;  2. Altered source versions must be plainly marked as such, and must not be
+;     misrepresented as being the original software.
+;  3. This notice may not be removed or altered from any source distribution.
+
+decompress_lzsa1
+lz1token ldb ,x+           ; load next token into B: O|LLL|MMMM
+         pshs b            ; save it
+
+         lda #$00          ; clear A (high part of literals count)
+         andb #$70         ; isolate LLL (embedded literals count) in B
+         beq lz1nolt       ; skip if no literals
+         cmpb #$70         ; LITERALS_RUN_LEN?
+         bne lz1declt      ; if not, we have the complete count, go unshift
+
+         ldb ,x+           ; load extra literals count byte
+         addb #$07         ; add LITERALS_RUN_LEN
+         bcc lz1gotlt      ; if no overflow, we got the complete count, copy
+         bne lz1midlt
+
+         ldb ,x+           ; load low 8 bits of little-endian literals count
+         lda ,x+           ; load high 8 bits of literal count
+         jmp lz1gotlt      ; we now have the complete count, go copy
+
+lz1midlt tfr b,a           ; copy high part of literals count into A
+         ldb ,x+           ; load low 8 bits of literals count
+         jmp lz1gotlt      ; we now have the complete count, go copy
+
+lz1declt lsrb              ; shift literals count into place
+         lsrb
+         lsrb
+         lsrb
+ 
+lz1gotlt tfr x,u
+         tfr d,x           ; transfer 16-bit count into X
+lz1cpylt lda ,u+           ; copy literal byte
+         sta ,y+ 
+         leax -1,x         ; decrement X and update Z flag
+         bne lz1cpylt      ; loop until all literal bytes are copied
+         tfr u,x
+
+lz1nolt  puls b            ; restore token
+         pshs b            ; save it again
+      
+         tstb              ; test O bit (small or large offset)
+         bmi lz1bigof
+
+         ldb ,x+           ; O clear: load 8 bit (negative, signed) offset
+         lda #$ff          ; set high 8 bits
+         jmp lz1gotof
+
+lz1bigof ldb ,x+           ; O set: load long 16 bit (negative, signed) offset
+         lda ,x+           ; (little endian)
+lz1gotof leau d,y          ; put backreference start address in U (dst+offset)
+         
+         puls b            ; restore token
+         
+         lda #$00          ; clear A (high part of match length)
+         andb #$0F         ; isolate MMMM (embedded match length)
+         addb #$03         ; add MIN_MATCH_SIZE
+         cmpb #$12         ; MATCH_RUN_LEN?
+         bne lz1gotln      ; no, we have the full match length, go copy
+
+         ldb ,x+           ; grab extra match length byte
+         addb #$12         ; add MIN_MATCH_SIZE + MATCH_RUN_LEN
+         bcc lz1gotln      ; if no overflow, we have the full length
+         bne lz1midln
+
+         ldb ,x+           ; load 16-bit len in D (low part in B, high in A)
+         lda ,x+           ; (little endian)
+         bne lz1gotln      ; check if we hit EOD (16-bit length = 0)
+         tstb
+         bne lz1gotln      ; go copy matched bytes if not
+      
+         rts               ; done, bail
+
+lz1midln tfr b,a           ; copy high part of len into A
+         ldb ,x+           ; grab low 8 bits of len in B
+
+lz1gotln pshs x            ; save source compressed data pointer
+         tfr d,x           ; copy match length to X
+
+lz1cpymt lda ,u+           ; copy matched byte
+         sta ,y+ 
+         leax -1,x         ; decrement X
+         bne lz1cpymt      ; loop until all matched bytes are copied
+
+         puls x            ; restore source compressed data pointer
+         jmp lz1token      ; go decode next token