quartz/content/notes/07-6809-advanced.md

title	aliases	tags
07-6809-advanced		lecture cosc204

addressing modes

• implied inca
• immediate lda #$00
• extended LDA $31FE
• extended indirect LDA [$31FE] or LDA [HERE] points to a memory location which specifies another memory location
• direct LDA $ff or lda <HERE
• register trf X,Y
• indexed LDA $3111E, X use memory location that is the sum of the operands
  • LDA ,X
  • LDA $23,X
  • etc
• indexed indirect LDA [$10, X]
• relative BEQ HERE
  • BEQ does a short branch
  • LBEQ does a long branch
  • the value is relative to HERE
• program counter relative LDA TABLE,PCR ; A = the value stored at TABLE
  • 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

Calling conventions

all global vars. this is discouraged in high level languages

The calling convention is the set of rules that describe

• How to pass parameters
• How to return a result
• Which registers a routine may alter

CMOC

a 6809 C compiler

callling conventions

• A routine must preserve Y, U, S and DP

• A routine may change A, B, X and CC

• Parameters are pushed on the stack in the reverse order

• The caller pops them off the stack after the call

• char parameters are promoted to int

• unsigned char are promoted to unsigned int

• Return 8-but values in B

• Return 16-bit values in D

• We’re not going to talk about passing a struct by value

example in C

uint16_t two_params(uint8_t first, uint16_t second) { 
	return first + second; 
} 

uint16_t call_one(void) { 
	return two_params(204, 431); 
}

LDD #$01AF ;
decimal 431 PSHS B,A ;
argument 2, int CLRA LDB #$CC ;
decimal 204 PSHS B,A ;
argument 1, int LBSR _two_params