decoder.s

| Assembly Source File
| Created 8/16/09; 23:25:14

||Refer to http://www.z80.info/decoding.htm
||General things:
|============useful definitions=============
AMS_jumptable = 0xC8

|============text segment (code)============
.text
.xdef asm_main

|Helper for LUTs- just throw the index in d0 and pass the label for the LUT
|LUT format: entries are 16-bit offsets from the address of the entry
|Only the lowest 6 bits in d0 mean anything- set bit 6 will screw things over
.macro goLUT lut
	lea \lut(PC), a0
	lsl.w #1, d0	|shift is cheaper than multiply
	adda.w (a0, d0.b), a0
	jsr a0
.endm
.macro goLUTb lut	|goLUT, but uses jmp rather than jsr
	lea \lut(PC), a0
	lsl.w #1, d0
	adda.w (a0, d0.b), a0
	jmp a0
.endm

|global registers:
|a5 is emulated PC (gotcha: need to handle when this gets outside the z80's
| address range (can I throw an exception when a5 is past a point?)
|a6-> cpuState structure
asm_main:
	| get a5 and a6 initialized from the caller
	|gogogo!

	rts

|decode handlers may use any registers they wish, excepting the globals (a5-a7)
|Instruction to decode is passed in on d7, and handlers
| should return the T-state count in d7.
|a5 is set to the byte following what was gotting into d7, and should point to the
| next instruction when returning.
decodeInstr:	|instruction is passed as d7 (clear except for lsb)
	move.b d7,d0
	lsr #6,d0
	goLUT _xLUT
	|do something else

|Checks emulated condition, returns nz if condition is true.
|d7 contains condition code
checkCondition:
	lsr.b #1, d7	|bit 0 of condition is reset to negate condition- get in C
	move.b _conditionsLUT(PC, d7.b), d7
	btst.b d7, reg_F(a6)	|check emulated flag
	bcs _checkCondition_noInvert
	eor.b #4, ccr	|flip Z flag
_checkCondition_noInvert:
	rts

|============data segment===================
 .data
 .even
_conditionsLUT:	|table of bit numbers for flags in z80
	.byte 6	|Z
	.byte 0	|C
	.byte 2	|P/V
	.byte 7	|S

_xLUT:	|LUT for handlers of X-values
	.word _hX0
	.word _hX1
	.word _hX2
	.word _hX3

_X0ZLUT:
	.word _hX0Z0
	.word _hX0Z1
	.word _hX0Z2
	.word _hX0Z3
	.word _hX0Z4
	.word _hX0Z5
	.word _hX0Z6
	.word _hX0Z7

_X0Z0LUT:
	.word _hX0Z0NOP
	.word _hX0Z0EXAF
	.word _hX0Z0DJNZ
	.word _hX0Z0JR