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

title	aliases	tags	sr-due	sr-interval	sr-ease
07-6809-advanced		lecture cosc204	2022-08-15	10	250

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 $311E, 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

parameters

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

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

local variables

uint16_t one_param(uint16_t xyzzy) { 
	uint16_t val = xyzzy; 
	return val; 
}

_one_param 
	PSHS U 
	LEAU ,S 
	LEAS -2,S 
	* Formal parameter(s): 
	* 4,U: 2 bytes: xyzzy 
	* Local non-static variable(s): 
	* -2,U: 2 bytes: val 
	LDD 4,U 
	STD -2,U 
	LEAS ,U 
	PULS U,PC