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; -----------------------------------------------------------------------------
; ZX7 decoder by Einar Saukas & Urusergi
; "Turbo" version (88 bytes, 25% faster)
; -----------------------------------------------------------------------------
; Parameters:
; HL: source address (compressed data)
; DE: destination address (decompressing)
; -----------------------------------------------------------------------------
dzx7_turbo:
ld a, #80
dzx7t_copy_byte_loop:
ldi ; copy literal byte
dzx7t_main_loop:
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
jr nc, dzx7t_copy_byte_loop ; next bit indicates either literal or sequence
; determine number of bits used for length (Elias gamma coding)
push de
ld bc, 1
ld d, b
dzx7t_len_size_loop:
inc d
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
jr nc, dzx7t_len_size_loop
jp dzx7t_len_value_start
; determine length
dzx7t_len_value_loop:
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
rl c
rl b
jr c, dzx7t_exit ; check end marker
dzx7t_len_value_start:
dec d
jr nz, dzx7t_len_value_loop
inc bc ; adjust length
; determine offset
ld e, (hl) ; load offset flag (1 bit) + offset value (7 bits)
inc hl
defb #cb, #33 ; opcode for undocumented instruction "SLL E" aka "SLS E"
jr nc, dzx7t_offset_end ; if offset flag is set, load 4 extra bits
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
rl d ; insert first bit into D
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
rl d ; insert second bit into D
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
rl d ; insert third bit into D
add a, a ; check next bit
call z, dzx7t_load_bits ; no more bits left?
ccf
jr c, dzx7t_offset_end
inc d ; equivalent to adding 128 to DE
dzx7t_offset_end:
rr e ; insert inverted fourth bit into E
; copy previous sequence
ex (sp), hl ; store source, restore destination
push hl ; store destination
sbc hl, de ; HL = destination - offset - 1
pop de ; DE = destination
ldir
dzx7t_exit:
pop hl ; restore source address (compressed data)
jp nc, dzx7t_main_loop
dzx7t_load_bits:
ld a, (hl) ; load another group of 8 bits
inc hl
rla
ret
; -----------------------------------------------------------------------------
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