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118 lines
1.9 KiB
Markdown
118 lines
1.9 KiB
Markdown
---
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title: "06-6809-programming"
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aliases:
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tags:
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- lecture
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- cosc204
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---
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difficult
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- no variables
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- GOTO not loops
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- only compare ints and bytes
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- not really routines
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- all vars global
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- case sensitive
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# Hello World
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- need a routine that given a string will print each chracter one at a time
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- need to know how to print
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## Routines
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```
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routine:
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rts
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```
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- no params
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- no local vars
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calling routine
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- lbsr, bsr, jsr
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- very subtle differences
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- we use bsr (branch to subroutine)
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```
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bsr routine
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```
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## Interation
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use 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
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beq done //if equal, 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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- comparison only compares and sets flags in CC
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- you then branch on the result
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- `beq bge` etc
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- other intructions set flags to
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- `lda` etc
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- so
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```
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lda $45
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beq somewhere
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```
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## 6809.uk text screen
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write h to top left
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```
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lda #'H'
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sta $0400
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```
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add an 'e' beside that
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```
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lda #'E'
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sta $0401
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```
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## print routine
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- pass the address of the string in Y
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- "address" - the index into the memory array the hold the first character of the string
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- use $00 to end the string
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## algorithm
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- print a string
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- Load a character from memory\[Y]
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- Move on to the next character (Y++)
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- Compare the character to $00 If equal exit this routine
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- Write the character to the screen
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- Move on to the next screen location
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- GOTO start
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PUTS()
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```
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;
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;
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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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PUTS:
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ldx #$0400 ; start of screen
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more:
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lda 0,y ; current char -> A (load y into a)
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leay 1,y ; increment y
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cmpa #$00 ; was it a zero?
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beq done ; if it was 0 then return
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sta 0,x ; write (store a in x)
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leax 1,x ; increment x
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bra more ; repeat
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done:
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rts ; return from this routine
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```
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