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97 lines
2.0 KiB
Markdown
97 lines
2.0 KiB
Markdown
---
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title: "07-6809-advanced"
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aliases:
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tags:
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- lecture
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- cosc204
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sr-due: 2022-08-15
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sr-interval: 10
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sr-ease: 250
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---
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# addressing modes
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- implied `inca`
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- immediate `lda #$00`
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- extended `LDA $31FE`
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- extended indirect `LDA [$31FE]` or `LDA [HERE]` points to a memory location which specifies another memory location
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- direct `LDA $ff` or `lda <HERE`
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- register `trf X,Y`
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- indexed `LDA $311E, X` use memory location that is the sum of the operands
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- `LDA ,X`
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- `LDA $23,X`
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- etc
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- indexed indirect `LDA [$10, X]`
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- relative `BEQ HERE`
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- BEQ does a short branch
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- LBEQ does a long branch
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- the value is relative to HERE
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- program counter relative `LDA TABLE,PCR ; A = the value stored at TABLE`
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- if all memory references are relative, then the program cna be loaded anywhere is memory, and will still work. It is said to be **relocatable**
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# Calling conventions
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all global vars. this is discouraged in high level languages
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The calling convention is the set of rules that describe
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- How to pass parameters
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- How to return a result
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- Which registers a routine may alter
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## CMOC
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a 6809 C compiler
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callling conventions
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- A routine must preserve Y, U, S and DP
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- A routine may change A, B, X and CC
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- Parameters are pushed on the stack in the reverse order
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- The caller pops them off the stack after the call
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- char parameters are promoted to int
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- unsigned char are promoted to unsigned int
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- Return 8-but values in B
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- Return 16-bit values in D
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- We’re not going to talk about passing a struct by value
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### example in C
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parameters
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``` c
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uint16_t two_params(uint8_t first, uint16_t second) {
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return first + second;
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}
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uint16_t call_one(void) {
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return two_params(204, 431);
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}
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```
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```
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```
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local variables
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``` c
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uint16_t one_param(uint16_t xyzzy) {
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uint16_t val = xyzzy;
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return val;
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}
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```
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```
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_one_param
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PSHS U
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LEAU ,S
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LEAS -2,S
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* Formal parameter(s):
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* 4,U: 2 bytes: xyzzy
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* Local non-static variable(s):
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* -2,U: 2 bytes: val
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LDD 4,U
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STD -2,U
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LEAS ,U
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PULS U,PC
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```
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