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assembler.asm
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assembler.asm
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; Assembler for Tali Forth 2
; Scot W. Stevenson <scot.stevenson@gmail.com>
; First version: 07. Nov 2014 (as tasm65c02)
; This version: 03. Jan 2019
; This is the built-in assembler for Tali Forth 2. Once the assembler wordlist
; is included with
; assembler-wordlist >order
; the opcodes are available as normal Forth words. The format is Simpler
; Assembler Notation (SAN) which separates the opcode completely from the
; operand (see https://github.com/scotws/SAN). In this case, the operand is
; entered before the opcode in the postfix Forth notation (for example, "2000
; lda.#"). See the assembler documenation in the manual for more detail.
; The code here was originally used in A Typist's Assembler for the 65c02
; (tasm65c02), see https://github.com/scotws/tasm65c02 for the standalone
; version. Tasm65c02 is in the public domain.
; This code makes use of the opcode tables stored as part of the disassembler.
; ==========================================================
; MNEMONICS
; The assembler instructions are realized as individual Forth words with
; entries in the assembler wordlist (see header.asm). We pass the opcode in A.
; An alterantive method would have been along the lines of
; jsr asm_common
; .byte $EA
; where the asm_common then uses the address on the Return Stack to pick up the
; opcode and the length. Though this uses fewer resources, the current version
; makes up for this by simplifying the code of asm_common.
; The routines are organized alphabetically by SAN mnemonic, not by opcode. The
; SAN and traditional mnemonics are listed after the opcode load instruction.
; This list was generated by a Python script in the tools folder, see there for
; more detail.
assembler: ; used to calculate size of assembler code
xt_asm_adc_h: ; adc.# \ ADC #nn
lda #$69
jmp asm_common
z_asm_adc_h:
xt_asm_adc_x: ; adc.x \ ADC nnnn,X
lda #$7D
jmp asm_common
z_asm_adc_x:
xt_asm_adc_y: ; adc.y \ ADC nnnn,Y
lda #$79
jmp asm_common
z_asm_adc_y:
xt_asm_adc_z: ; adc.z \ ADC nn
lda #$65
jmp asm_common
z_asm_adc_z:
xt_asm_adc_zi: ; adc.zi \ ADC (nn)
lda #$72
jmp asm_common
z_asm_adc_zi:
xt_asm_adc_ziy: ; adc.ziy \ ADC (nn),Y
lda #$71
jmp asm_common
z_asm_adc_ziy:
xt_asm_adc_zx: ; adc.zx \ ADC nn,X
lda #$75
jmp asm_common
z_asm_adc_zx:
xt_asm_adc_zxi: ; adc.zxi \ ADC (nn,X)
lda #$61
jmp asm_common
z_asm_adc_zxi:
xt_asm_and: ; and. \ AND nnnn
lda #$2D
jmp asm_common
z_asm_and:
xt_asm_and_h: ; and.# \ AND #nn
lda #$29
jmp asm_common
z_asm_and_h:
xt_asm_and_x: ; and.x \ AND nnnn,X
lda #$3D
jmp asm_common
z_asm_and_x:
xt_asm_and_y: ; and.y \ AND nnnn,Y
lda #$39
jmp asm_common
z_asm_and_y:
xt_asm_and_z: ; and.z \ AND nn
lda #$25
jmp asm_common
z_asm_and_z:
xt_asm_and_zi: ; and.zi \ AND (nn)
lda #$32
jmp asm_common
z_asm_and_zi:
xt_asm_and_ziy: ; and.ziy \ AND (nn),Y
lda #$31
jmp asm_common
z_asm_and_ziy:
xt_asm_and_zx: ; and.zx \ AND nn,X
lda #$35
jmp asm_common
z_asm_and_zx:
xt_asm_and_zxi: ; and.zxi \ AND (nn,X)
lda #$21
jmp asm_common
z_asm_and_zxi:
xt_asm_asl: ; asl \ ASL nnnn
lda #$0E
jmp asm_common
z_asm_asl:
xt_asm_asl_a: ; asl.a \ ASL
lda #$0A
jmp asm_common
z_asm_asl_a:
xt_asm_asl_x: ; asl.x \ ASL nnnn,X
lda #$1E
jmp asm_common
z_asm_asl_x:
xt_asm_asl_z: ; asl.z \ ASL nn
lda #$06
jmp asm_common
z_asm_asl_z:
xt_asm_asl_zx: ; asl.zx \ ASL nn,X
lda #$16
jmp asm_common
z_asm_asl_zx:
xt_asm_bcc: ; bcc \ BCC
lda #$90
jmp asm_common
z_asm_bcc:
xt_asm_bcs: ; bcs \ BCS
lda #$B0
ldy #2
jmp asm_common
z_asm_bcs:
xt_asm_beq: ; beq \ BEQ
lda #$F0
jmp asm_common
z_asm_beq:
xt_asm_bit: ; bit \ BIT nnnn
lda #$2C
jmp asm_common
z_asm_bit:
xt_asm_bit_h: ; bit.# \ BIT #nn
lda #$89
jmp asm_common
z_asm_bit_h:
xt_asm_bit_x: ; bit.x \ BIT nnnn,X
lda #$3C
jmp asm_common
z_asm_bit_x:
xt_asm_bit_z: ; bit.z \ BIT nn
lda #$24
jmp asm_common
z_asm_bit_z:
xt_asm_bit_zx: ; bit.zx \ BIT nn,X
lda #$34
jmp asm_common
z_asm_bit_zx:
xt_asm_bmi: ; bmi \ BMI
lda #$30
jmp asm_common
z_asm_bmi:
xt_asm_bne: ; bne \ BNE
lda #$D0
jmp asm_common
z_asm_bne:
xt_asm_bpl: ; bpl \ BPL
lda #$10
jmp asm_common
z_asm_bpl:
xt_asm_bra: ; bra \ BRA
lda #$80
jmp asm_common
z_asm_bra:
xt_asm_brk: ; brk \ BRK
lda #$00
jmp asm_common
z_asm_brk:
xt_asm_bvc: ; bvc \ BVC
lda #$50
jmp asm_common
z_asm_bvc:
xt_asm_bvs: ; bvs \ BVS
lda #$70
jmp asm_common
z_asm_bvs:
xt_asm_clc: ; clc \ CLC
lda #$18
jmp asm_common
z_asm_clc:
xt_asm_cld: ; cld \ CLD
lda #$D8
jmp asm_common
z_asm_cld:
xt_asm_cli: ; cli \ CLI
lda #$58
jmp asm_common
z_asm_cli:
xt_asm_clv: ; clv \ CLV
lda #$B8
jmp asm_common
z_asm_clv:
xt_asm_cmp: ; cmp \ CMP nnnn
lda #$CD
jmp asm_common
z_asm_cmp:
xt_asm_cmp_h: ; cmp.# \ CMP #nn
lda #$C9
jmp asm_common
z_asm_cmp_h:
xt_asm_cmp_x: ; cmp.x \ CMP nnnn,X
lda #$DD
jmp asm_common
z_asm_cmp_x:
xt_asm_cmp_y: ; cmp.y \ CMP nnnn,Y
lda #$D9
jmp asm_common
z_asm_cmp_y:
xt_asm_cmp_z: ; cmp.z \ CMP nn
lda #$C5
jmp asm_common
z_asm_cmp_z:
xt_asm_cmp_zi: ; cmp.zi \ CMP (nn)
lda #$D2
jmp asm_common
z_asm_cmp_zi:
xt_asm_cmp_ziy: ; cmp.ziy \ CMP (nn),Y
lda #$D1
jmp asm_common
z_asm_cmp_ziy:
xt_asm_cmp_zx: ; cmp.zx \ CMP nn,X
lda #$D5
jmp asm_common
z_asm_cmp_zx:
xt_asm_cmp_zxi: ; cmp.zxi \ CMP (nn,X)
lda #$C1
jmp asm_common
z_asm_cmp_zxi:
xt_asm_cpx: ; cpx \ CPX nnnn
lda #$EC
jmp asm_common
z_asm_cpx:
xt_asm_cpx_h: ; cpx.# \ CPX #nn
lda #$E0
jmp asm_common
z_asm_cpx_h:
xt_asm_cpx_z: ; cpx.z \ CPX nn
lda #$E4
jmp asm_common
z_asm_cpx_z:
xt_asm_cpy: ; cpy \ CPY
lda #$CC
ldy #3
jmp asm_common
z_asm_cpy:
xt_asm_cpy_h: ; cpy.# \ CPY #nn
lda #$C0
jmp asm_common
z_asm_cpy_h:
xt_asm_cpy_z: ; cpy.z \ CPY nn
lda #$C4
jmp asm_common
z_asm_cpy_z:
xt_asm_dec: ; dec \ DEC nnnn
lda #$CE
jmp asm_common
z_asm_dec:
xt_asm_dec_a: ; dec.a \ DEC
lda #$3A
jmp asm_common
z_asm_dec_a:
xt_asm_dec_x: ; dec.x \ DEC nnnn,X
lda #$DE
jmp asm_common
z_asm_dec_x:
xt_asm_dec_z: ; dec.z \ DEC nn
lda #$C6
jmp asm_common
z_asm_dec_z:
xt_asm_dec_zx: ; dec.zx \ DEC nn,X
lda #$D6
jmp asm_common
z_asm_dec_zx:
xt_asm_dex: ; dex \ DEX
lda #$CA
jmp asm_common
z_asm_dex:
xt_asm_dey: ; dey \ DEY
lda #$88
jmp asm_common
z_asm_dey:
xt_asm_eor: ; eor \ EOR nnnn
lda #$4D
jmp asm_common
z_asm_eor:
xt_asm_eor_h: ; eor.# \ EOR #nn
lda #$49
jmp asm_common
z_asm_eor_h:
xt_asm_eor_x: ; eor.x \ EOR nnnn,X
lda #$5D
jmp asm_common
z_asm_eor_x:
xt_asm_eor_y: ; eor.y \ EOR nnnn,Y
lda #$59
jmp asm_common
z_asm_eor_y:
xt_asm_eor_z: ; eor.z \ EOR nn
lda #$45
jmp asm_common
z_asm_eor_z:
xt_asm_eor_zi: ; eor.zi \ EOR (nn)
lda #$52
jmp asm_common
z_asm_eor_zi:
xt_asm_eor_ziy: ; eor.ziy \ EOR (nn),Y
lda #$51
jmp asm_common
z_asm_eor_ziy:
xt_asm_eor_zx: ; eor.zx \ EOR nn,X
lda #$55
jmp asm_common
z_asm_eor_zx:
xt_asm_eor_zxi: ; eor.zxi \ EOR (nn,X)
lda #$41
jmp asm_common
z_asm_eor_zxi:
xt_asm_inc: ; inc \ INC nnnn
lda #$EE
jmp asm_common
z_asm_inc:
xt_asm_inc_a: ; inc.a \ INC
lda #$1A
jmp asm_common
z_asm_inc_a:
xt_asm_inc_x: ; inc.x \ INC nnnn,X
lda #$FE
jmp asm_common
z_asm_inc_x:
xt_asm_inc_z: ; inc.z \ INC nn
lda #$E6
jmp asm_common
z_asm_inc_z:
xt_asm_inc_zx: ; inc.zx \ INC nn,X
lda #$F6
jmp asm_common
z_asm_inc_zx:
xt_asm_inx: ; inx \ INX
lda #$E8
jmp asm_common
z_asm_inx:
xt_asm_iny: ; iny \ INY
lda #$C8
jmp asm_common
z_asm_iny:
xt_asm_jmp: ; jmp \ JMP nnnn
lda #$4C
jmp asm_common
z_asm_jmp:
xt_asm_jmp_i: ; jmp.i \ JMP (nnnn)
lda #$6C
jmp asm_common
z_asm_jmp_i:
xt_asm_jmp_xi: ; jmp.xi \ JMP (nnnn,X)
lda #$7C
jmp asm_common
z_asm_jmp_xi:
xt_asm_jsr: ; jsr \ JSR nnnn
lda #$20
jmp asm_common
z_asm_jsr:
xt_asm_lda: ; lda \ LDA nnnn
lda #$AD
jmp asm_common
z_asm_lda:
xt_asm_lda_h: ; lda.# \ LDA #nn
lda #$A9
jmp asm_common
z_asm_lda_h:
xt_asm_lda_x: ; lda.x \ LDA nnnn,X
lda #$BD
jmp asm_common
z_asm_lda_x:
xt_asm_lda_y: ; lda.y \ LDA nnnn,Y
lda #$B9
jmp asm_common
z_asm_lda_y:
xt_asm_lda_z: ; lda.z \ LDA nn
lda #$A5
jmp asm_common
z_asm_lda_z:
xt_asm_lda_zi: ; lda.zi \ LDA (nn)
lda #$B2
jmp asm_common
z_asm_lda_zi:
xt_asm_lda_ziy: ; lda.ziy \ LDA (nn),Y
lda #$B1
jmp asm_common
z_asm_lda_ziy:
xt_asm_lda_zx: ; lda.zx \ LDA nn,X
lda #$B5
jmp asm_common
z_asm_lda_zx:
xt_asm_lda_zxi: ; lda.zxi \ LDA (nn,X)
lda #$A1
jmp asm_common
z_asm_lda_zxi:
xt_asm_ldx: ; ldx \ LDX nnnn
lda #$AE
jmp asm_common
z_asm_ldx:
xt_asm_ldx_h: ; ldx.# \ LDX #nn
lda #$A2
jmp asm_common
z_asm_ldx_h:
xt_asm_ldx_y: ; ldx.y \ LDX nnnn,Y
lda #$BE
jmp asm_common
z_asm_ldx_y:
xt_asm_ldx_z: ; ldx.z \ LDX nn
lda #$A6
jmp asm_common
z_asm_ldx_z:
xt_asm_ldx_zy: ; ldx.zy \ LDX nn,Y
lda #$B6
jmp asm_common
z_asm_ldx_zy:
xt_asm_ldy: ; ldy \ LDY nnnn
lda #$AC
jmp asm_common
z_asm_ldy:
xt_asm_ldy_h: ; ldy.# \ LDY #nn
lda #$A0
jmp asm_common
z_asm_ldy_h:
xt_asm_ldy_x: ; ldy.x \ LDY nnnn,X
lda #$BC
jmp asm_common
z_asm_ldy_x:
xt_asm_ldy_z: ; ldy.z \ LDY nn
lda #$A4
jmp asm_common
z_asm_ldy_z:
xt_asm_ldy_zx: ; ldy.zx \ LDY nn,X
lda #$B4
jmp asm_common
z_asm_ldy_zx:
xt_asm_lsr: ; lsr \ LSR nnnn
lda #$4E
jmp asm_common
z_asm_lsr:
xt_asm_lsr_a: ; lsr.a \ LSR
lda #$4A
jmp asm_common
z_asm_lsr_a:
xt_asm_lsr_x: ; lsr.x \ LSR nnnn,X
lda #$5E
jmp asm_common
z_asm_lsr_x:
xt_asm_lsr_z: ; lsr.z \ LSR nn
lda #$46
jmp asm_common
z_asm_lsr_z:
xt_asm_lsr_zx: ; lsr.zx \ LSR nn,X
lda #$56
jmp asm_common
z_asm_lsr_zx:
xt_asm_nop: ; nop \ NOP
lda #$EA
jmp asm_common
z_asm_nop:
xt_asm_ora: ; ora \ ORA nnnn
lda #$0D
jmp asm_common
z_asm_ora:
xt_asm_ora_h: ; ora.# \ ORA #nn
lda #$09
jmp asm_common
z_asm_ora_h:
xt_asm_ora_x: ; ora.x \ ORA nnnn,X
lda #$1D
jmp asm_common
z_asm_ora_x:
xt_asm_ora_y: ; ora.y \ ORA nnnn,Y
lda #$19
jmp asm_common
z_asm_ora_y:
xt_asm_ora_z: ; ora.z \ ORA nn
lda #$05
jmp asm_common
z_asm_ora_z:
xt_asm_ora_zi: ; ora.zi \ ORA.ZI
lda #$12
ldy #2
jmp asm_common
z_asm_ora_zi:
xt_asm_ora_ziy: ; ora.ziy \ ORA (nn),Y
lda #$11
jmp asm_common
z_asm_ora_ziy:
xt_asm_ora_zx: ; ora.zx \ ORA nn,X
lda #$15
jmp asm_common
z_asm_ora_zx:
xt_asm_ora_zxi: ; ora.zxi \ ORA (nn,X)
lda #$01
jmp asm_common
z_asm_ora_zxi:
xt_asm_pha: ; pha \ PHA
lda #$48
jmp asm_common
z_asm_pha:
xt_asm_php: ; php \ PHP
lda #$08
jmp asm_common
z_asm_php:
xt_asm_phx: ; phx \ PHX
lda #$DA
jmp asm_common
z_asm_phx:
xt_asm_phy: ; phy \ PHY
lda #$5A
jmp asm_common
z_asm_phy:
xt_asm_pla: ; pla \ PLA
lda #$68
jmp asm_common
z_asm_pla:
xt_asm_plp: ; plp \ PLP
lda #$28
jmp asm_common
z_asm_plp:
xt_asm_plx: ; plx \ PLX
lda #$FA
jmp asm_common
z_asm_plx:
xt_asm_ply: ; ply \ PLY
lda #$7A
jmp asm_common
z_asm_ply:
xt_asm_rol: ; rol \ ROL nnnn
lda #$2E
jmp asm_common
z_asm_rol:
xt_asm_rol_a: ; rol.a \ ROL
lda #$2A
jmp asm_common
z_asm_rol_a:
xt_asm_rol_x: ; rol.x \ ROL nnnn,X
lda #$3E
jmp asm_common
z_asm_rol_x:
xt_asm_rol_z: ; rol.z \ ROL nn
lda #$26
jmp asm_common
z_asm_rol_z:
xt_asm_rol_zx: ; rol.zx \ ROL nn,X
lda #$36
jmp asm_common
z_asm_rol_zx:
xt_asm_ror: ; ror \ ROR nnnn
lda #$6E
jmp asm_common
z_asm_ror:
xt_asm_ror_a: ; ror.a \ ROR
lda #$6A
jmp asm_common
z_asm_ror_a:
xt_asm_ror_x: ; ror.x \ ROR nnnn,X
lda #$7E
jmp asm_common
z_asm_ror_x:
xt_asm_ror_z: ; ror.z \ ROR nn
lda #$66
jmp asm_common
z_asm_ror_z:
xt_asm_ror_zx: ; ror.zx \ ROR nn,X
lda #$76
jmp asm_common
z_asm_ror_zx:
xt_asm_rti: ; rti \ RTI
lda #$40
jmp asm_common
z_asm_rti:
xt_asm_rts: ; rts \ RTS
lda #$60
jmp asm_common
z_asm_rts:
xt_asm_sbc: ; sbc \ SBC nnnn
lda #$ED
jmp asm_common
z_asm_sbc:
xt_asm_sbc_h: ; sbc.# \ SBC #nn
lda #$E9
jmp asm_common
z_asm_sbc_h:
xt_asm_sbc_x: ; sbc.x \ SBC nnnn,X
lda #$FD
jmp asm_common
z_asm_sbc_x:
xt_asm_sbc_y: ; sbc.y \ SBC nnnn,Y
lda #$F9
jmp asm_common
z_asm_sbc_y:
xt_asm_sbc_z: ; sbc.z \ SBC nn
lda #$E5
jmp asm_common
z_asm_sbc_z:
xt_asm_sbc_zi: ; sbc.zi \ SBC (nn)
lda #$F2
jmp asm_common
z_asm_sbc_zi:
xt_asm_sbc_ziy: ; sbc.ziy \ SBC (nn),Y
lda #$F1
jmp asm_common
z_asm_sbc_ziy:
xt_asm_sbc_zx: ; sbc.zx \ SBC nn,X
lda #$F5
jmp asm_common
z_asm_sbc_zx:
xt_asm_sbc_zxi: ; sbc.zxi \ SBC (nn,X)
lda #$E1
bra asm_common ; <-- limit for BRA instead of JMP
z_asm_sbc_zxi:
xt_asm_sec: ; sec \ SEC
lda #$38
bra asm_common
z_asm_sec:
xt_asm_sed: ; sed \ SED
lda #$F8
bra asm_common
z_asm_sed:
xt_asm_sei: ; sei \ SEI
lda #$78
bra asm_common
z_asm_sei:
xt_asm_sta: ; sta \ STA nnnn
lda #$8D
bra asm_common
z_asm_sta:
xt_asm_sta_x: ; sta.x \ STA nnnn,X
lda #$9D
bra asm_common
z_asm_sta_x:
xt_asm_sta_y: ; sta.y \ STA nnnn,Y
lda #$99
bra asm_common
z_asm_sta_y:
xt_asm_sta_z: ; sta.z \ STA nn
lda #$85
bra asm_common
z_asm_sta_z:
xt_asm_sta_zi: ; sta.zi \ STA (nn)
lda #$92
bra asm_common
z_asm_sta_zi:
xt_asm_sta_ziy: ; sta.ziy \ STA (nn),Y
lda #$91
bra asm_common
z_asm_sta_ziy:
xt_asm_sta_zx: ; sta.zx \ STA nn,X
lda #$95
bra asm_common
z_asm_sta_zx:
xt_asm_sta_zxi: ; sta.zxi \ STA (nn,X)
lda #$81
bra asm_common
z_asm_sta_zxi:
xt_asm_stx: ; stx \ STX nnnn
lda #$8E
bra asm_common
z_asm_stx:
xt_asm_stx_z: ; stx.z \ STX nn
lda #$86
bra asm_common
z_asm_stx_z:
xt_asm_stx_zy: ; stx.zy \ STX nn,Y
lda #$96
bra asm_common
z_asm_stx_zy:
xt_asm_sty: ; sty \ STY nnnn
lda #$8C
bra asm_common
z_asm_sty:
xt_asm_sty_z: ; sty.z \ STY nn
lda #$84
bra asm_common
z_asm_sty_z:
xt_asm_sty_zx: ; sty.zx \ STY nn,X
lda #$94
bra asm_common
z_asm_sty_zx:
xt_asm_stz: ; stz \ STZ nnnn
lda #$9C
bra asm_common
z_asm_stz:
xt_asm_stz_x: ; stz.x \ STZ nnnn,X
lda #$9E
bra asm_common
z_asm_stz_x:
xt_asm_stz_z: ; stz.z \ STZ nn
lda #$64
bra asm_common
z_asm_stz_z:
xt_asm_stz_zx: ; stz.zx \ STZ nn,X
lda #$74
bra asm_common
z_asm_stz_zx:
xt_asm_tax: ; tax \ TAX
lda #$AA
bra asm_common
z_asm_tax:
xt_asm_tay: ; tay \ TAY
lda #$A8
bra asm_common
z_asm_tay:
xt_asm_trb: ; trb \ TRB nnnn
lda #$1C
bra asm_common
z_asm_trb:
xt_asm_trb_z: ; trb.z \ TRB nn
lda #$14
bra asm_common
z_asm_trb_z:
xt_asm_tsb: ; tsb \ TSB nnnn
lda #$0C
bra asm_common
z_asm_tsb:
xt_asm_tsb_z: ; tsb.z \ TSB nn
lda #$04
bra asm_common
z_asm_tsb_z:
xt_asm_tsx: ; tsx \ TSX
lda #$BA
bra asm_common
z_asm_tsx:
xt_asm_txa: ; txa \ TXA
lda #$8A
bra asm_common
z_asm_txa:
xt_asm_txs: ; txs \ TXS
lda #$9A
bra asm_common
z_asm_txs:
xt_asm_tya: ; tya \ TYA
lda #$98
bra asm_common
z_asm_tya:
; ==========================================================
; ASSEMBLER HELPER FUNCTIONS
asm_common:
.scope
; """Common routine for all opcodes. We arrive here with the opcode in
; A. We do not need to check for the correct values because we are
; coming from the assembler Dictionary and trust our external test
; suite.
; """
; Compile opcode. Note cmpl_a does not use Y
tay
jsr cmpl_a
; We get the length of the opcode from the table included in
; the disassembler. We use the opcode value as the offset in
; the oc_index_table. We have 256 entries, each two bytes
; long, so we can't just use an index with Y. We use tmp2 for
; this.
lda #<oc_index_table
sta tmp2
lda #>oc_index_table
sta tmp2+1
tya ; retrieve opcode
asl ; times two for offset
bcc+
inc tmp2+1
*
tay ; use Y as the index
; Get address of the entry in the opcode table. We put it in
; tmp3 and push a copy of it to the stack to be able to print
; the opcode later
lda (tmp2),y ; LSB
sta tmp3
iny
lda (tmp2),y ; MSB
sta tmp3+1
lda (tmp3) ; get "lengths byte"
; The length of the instruction is stored in bits 7 and 6.
; Rotate them through the carry flag and mask the rest
rol
rol
rol ; Three times because we go through Carry
and #%00000011
tay
; One byte means no operand, we're done. Use DEY as CPY #1
dey
beq _done
; We have an operand which must be TOS
jsr underflow_1
; We compile the LSB of TOS as the operand we definitely have
; before we even test if this is a two- or three-byte
; instruction. Little endian CPU means we store this byte first
lda 0,x
jsr cmpl_a ; does not use Y
; If this is a two-byte instruction, we're done. If we landed
; here, we've already decremented Y by one, so this is
; the equivalent to CPY #2