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Tweak to ilen routine, shortened it a bit.

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This commit is contained in:
Rob Greene 2015-09-29 09:58:55 -05:00
parent cde7393fb5
commit 3deaa5a482
2 changed files with 11 additions and 9 deletions
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2
shared/Makefile
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@ -6,6 +6,8 @@ VIRTUAL2 = /Applications/Virtual\ \]\[/Virtual\ \]\[.app/Contents/MacOS/Virtual\
all:
@echo "Please select a target."
ilen-test: ilen-test.bin
ilen-test.bin: ilen-test.asm
cl65 -o ilen-test.bin -t none --start-addr 0x2000 -l ilen-test.asm
cp $(TEMPLATE) ilen-test.po

18
shared/ilen.asm
View File

@ -7,18 +7,18 @@
;
; Calculate Instruction Length
; Based on observations in 65C02 data sheet:
; * Generally, length is based on LOW nibble
; * There are a few special cases (including X9 column)
; * Generally, length is based on LOW nibble:
; - X0..X7 = 2 bytes
; - X8..XB = 1 byte
; - XC..XF = 3 bytes
; * There are a few special cases (including X0 and X9 columns)
; Input: Acc. = instruction
; Output: Acc. = length
;
.proc ilen
bit #$0f
bne @notcol0
@column0:
bit #%10010000 ; Normal instructions have at least one of these bits set
beq @notnormal ; None are set
@notcol0: ; "Normal" values for ALL columns 0-F
bit #%10011111 ; Normal instructions have at least one of these bits set
beq @column0 ; None are set, column 0 special cases ($00 / $20 / $40 / $60)
@normal: ; "Normal" values for ALL columns 0-F
asl
asl
asl
@ -36,7 +36,7 @@
lda #3
rts
@notnormal:
@column0: ; Only special cases
cmp #$20 ; JSR ($20)
beq @three
; fall through for BRK ($00) / RTI ($40) / RTS ($60)