89 lines
2.3 KiB
ArmAsm
89 lines
2.3 KiB
ArmAsm
;; Pack a word using the simple compression scheme.
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;; This has to be the same implementation as in compress_text.c
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;; because we compress input lines of BASIC to do the tokenisation.
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;; This allows us to search through the compressed tokens directly
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;; using the compressed fragment of BASIC, and thus work out the
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;; token for a given piece of string.
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;; This also helps ensure that we have an implementation that is totally
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;; different to that of the original C64 BASIC, as well as likely
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;; saving a bit of space in the ROM, especially once we start implementing
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;; larger keyword vocabs for BASIC 7 or BASIC 10 compatibility, since the
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;; compression and decompression machinery is of fixed size.
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pack_word:
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;; Inputs:
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;; X = offset of string to pack. Address is $0200 + X
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;; tokenise_work1 = number of bytes to pack
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;; tokenise_work2 = packed length of string
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;; Outputs:
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;; Output is written to $0100 (bottom of stack).
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;; Memory modified:
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;; tokenise_work4 = temporary storage
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;; tokenise_work3 = flag for whether we have a nybl prepared or not.
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;; (This can also be checked on exit, to know whether the last byte
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;; is incomplete, which is important for searching for abbreviated
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;; key words.)
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;; Initialise internal variables
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lda #$00
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sta tokenise_work2
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sta tokenise_work3
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pack_char_loop:
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;; Get the next char
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* lda $0200,x
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;; Work out what type of character we have, and thus
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;; how we need to pack it.
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jsr get_packed_char_index
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sta $0100,x
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inx
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cpx tokenise_work1
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bne -
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lda tokenise_work1
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sta tokenise_work2
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rts
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get_packed_char_index:
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;; Find char in A in the packed char list, and return
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;; $1 - $E, $F1 - $FD if the charactar is abbreviable,
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;; or else return $00 if the character has to be stored literally.
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;; Y register and A are modified, as are processor flags.
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ldy #$00
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find_char_loop:
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cmp packed_message_chars,y
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bne not_a_match
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;; Is it a nybl abbreviable char?
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cpy #13
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bcs not_nybl_char
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;; Yes, so add 1 and return it
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iny
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tya
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rts
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not_nybl_char:
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cpy #27
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bcs not_a_match
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;; It's a $Fx character.
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;; Since the lowest index is $F, and it should be encoded as $F1
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;; we need to add $E2
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tya
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clc
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adc #$E2
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rts
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not_a_match:
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iny
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;; Loop until we find it, or we discover it isn't
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;; a character with a special encoding
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cpy #27
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bne find_char_loop
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;; Not a char that we can pack, so return 0
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lda #$00
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rts
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