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88 lines
2.7 KiB
Markdown
88 lines
2.7 KiB
Markdown
---
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title: "6809"
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aliases:
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tags:
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- cosc204
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---
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[cheatsheet](https://www.chibiakumas.com/6809/CheatSheet.pdf)
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# Routines
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All variables are global so there is no params or local vars.
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There are three way to "Branch" to a subroutine.
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- lbsr
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- bsr
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- jsr
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There are subtle differences but we will use bsr mostly
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```
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bsr myroutine:
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myroutine:
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rts
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```
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# Iteration
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Done using GOTOs
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```
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clra //A = 0
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more:
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cmpa #$06 //compare A to $06 (sets the equal flag in CC if true)
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beq done //if the equal flag is set, branch to done
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inca //increment a
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bra more //branch to more
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done:
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```
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other instructions set flags too
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# Input/Output
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The screen always displays whatever is in the "screens memory location" which is $0400
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``` 6809 assembly
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lda #'H' ; load the character (indicated by '') into the a register
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sta $0400 ; store the contents of the a register in memory at location $0400
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```
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## printing a whole string
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``` 6809 assembly
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message:
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.byte "HELLO WORLD", 0
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START:
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ldy #message ; load "message", into the y register
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bsr puts ; branch to the "puts" subroutine
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rts ; return
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; Routine: PUTS
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; Pass the address of the string in Y
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;
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screen EQU $0400 ; start of screen
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PUTS:
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pshs a,x,y ; save a, x, and y ; store the contents of the a, x, and y registers so they are not overwritten
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ldx #screen ; start of screen ; load the hex value of the memory location of the screen ($0400) into the x register
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more:
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lda 0,y+ ; current char -> A ; load the contents of the Y register into the A register and increment Y by one
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cmpa #$00 ; was it a zero? ; if the character loaded into A was a zero set the equal flag
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beq done ; if it was 0 then return ; if the equal flag is set, branch to the "done" subroutine
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sta 0,x+ ; write ; otherwise, store the contents of the A register into memory at the location (hex value) defined in the X register, and increment the hex value stored in the X register
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bra more ; repeat ; branch to the "more" subroutine as there are more character to write
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done:
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puls a,x,y ; restore a, x, y ; re load the stored values of the a, x, and y registers that were saved earlier
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rts ; return from this routine
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```
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# Assembler
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- compiles assemble to machine code
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- can also dissamble machine code
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assembler create a listing file that gives the location, machines lcode, and assembly of the program
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