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| ; FORTH | ||
| ; Alex Dumont | ||
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| ; Help & Reference: | ||
| ; [Bitwise, Day 35: Implementing Forth](https://www.youtube.com/watch?v=rlayTh3sjiw) | ||
| ; [Moving Forth: Part 1](https://www.bradrodriguez.com/papers/moving1.htm) | ||
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| ; IP: Next Instruction Pointer (IP)-->W | ||
| ; W : Address of the code to run | ||
| W .= $FE ; 2 bytes, an address | ||
| IP .= W -2 | ||
| DTOP .= IP-2 | ||
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| *= $8000 | ||
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| RES_vec | ||
| CLD ; clear decimal mode | ||
| LDX #$FF | ||
| TXS ; set the stack pointer | ||
| LDX #DTOP | ||
| CLI | ||
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| LDA #<lbl_test1 | ||
| STA IP | ||
| LDA #>lbl_test1 | ||
| STA IP+1 | ||
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| NEXT: .MACRO | ||
| ; (IP) --> W | ||
| LDY #0 | ||
| LDA (IP),y | ||
| STA W | ||
| INY | ||
| LDA (IP),y | ||
| STA W+1 | ||
| ; IP+2 --> IP | ||
| CLC | ||
| LDA IP | ||
| ADC #2 ; A<-A+2 | ||
| STA IP | ||
| BCC .skip | ||
| INC IP+1 | ||
| .skip: | ||
| ; JMP (W) | ||
| JMP (W) | ||
| .ENDM | ||
| NEXT | ||
| *= $4000 | ||
| lbl_test1: | ||
| .DW forth_prog | ||
| *= $4100 | ||
| forth_prog: | ||
| JMP do_COLON | ||
| .DW do_PUSH1 | ||
| .DW do_DUP | ||
| .DW do_DROP | ||
| .DW other_word | ||
| .DW do_DROP | ||
| .DW do_SEMI | ||
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| other_word: | ||
| JMP do_COLON | ||
| .DW do_LIT | ||
| .DW $0002 | ||
| .DW do_LIT | ||
| .DW $0103 | ||
| .DW do_PLUS | ||
| .DW do_SWAP | ||
| .DW do_SEMI | ||
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| do_COLON: ; COLON aka ENTER | ||
| ; push IP to Return Stack | ||
| LDA IP+1 ; HI | ||
| PHA | ||
| LDA IP ; LO | ||
| PHA | ||
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| ; W+3 --> IP | ||
| ; (Code at W was a JMP, so 3 bytes) | ||
| LDA W+1 | ||
| STA IP+1 | ||
| LDA W | ||
| ADC #3 | ||
| STA IP | ||
| BCC skip | ||
| INC IP+1 | ||
| skip: | ||
| NEXT | ||
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| ; SEMICOLON aka EXIT | ||
| do_SEMI: | ||
| ; POP IP from Return Stack | ||
| PLA | ||
| STA IP | ||
| PLA | ||
| STA IP+1 | ||
| ; NEXT | ||
| NEXT | ||
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| do_SWAP: | ||
| LDA 2,X | ||
| LDY 4,X | ||
| STY 2,X | ||
| STA 4,X | ||
| LDA 3,X | ||
| LDY 5,X | ||
| STY 3,X | ||
| STA 5,X | ||
| NEXT | ||
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| do_DROP: | ||
| INX | ||
| INX | ||
| NEXT | ||
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| ; FORTH Primitive words | ||
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| do_PUSH0: | ||
| STZ 0,x | ||
| STZ 1,x | ||
| DEX | ||
| DEX | ||
| NEXT | ||
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| do_PUSH1: | ||
| LDA #1 | ||
| STA 0,x | ||
| STZ 1,x | ||
| DEX | ||
| DEX | ||
| NEXT | ||
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| ; Push a literal word (2 bytes) | ||
| do_LIT: | ||
| ; (IP) points to literal | ||
| ; instead of next instruction ;) | ||
| LDY #0 | ||
| LDA (IP),y | ||
| STA 0,X | ||
| INY | ||
| LDA (IP),y | ||
| STA 1,X | ||
| DEX | ||
| DEX | ||
| ; Now advance IP | ||
| ; IP+2 --> IP | ||
| LDA IP | ||
| ADC #2 | ||
| STA IP | ||
| BCC skip1 | ||
| INC IP+1 | ||
| skip1: | ||
| NEXT | ||
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| do_DUP: | ||
| LDA 2,X | ||
| STA 0,X | ||
| LDA 3,X | ||
| STA 1,X | ||
| DEX | ||
| DEX | ||
| NEXT | ||
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| do_PLUS: | ||
| CLC | ||
| INX | ||
| INX | ||
| LDA 0,X | ||
| ADC 2,X | ||
| STA 2,X | ||
| LDA 1,X | ||
| ADC 3,X | ||
| STA 3,X | ||
| NEXT | ||
| msg .BYTE "Monitor v0", 0 | ||
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| IRQ_vec | ||
| NMI_vec | ||
| RTI | ||
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| *= $0200 | ||
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| BYTE .DS 1 | ||
| LEN .DS 1 ; Length of CMD | ||
| CMD .DS 16 ; CMD string | ||
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| ; system vectors | ||
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| *= $FFFA | ||
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| .word NMI_vec ; NMI vector | ||
| .word RES_vec ; RESET vector | ||
| .WORD IRQ_vec ; IRQ vector |