m68tmr.c

#include "m68tmr.h"
#include <assert.h>
#include <stdio.h>

enum{
	TCR_RESPRE=3,
	TCR_EXTEN=4,
	TCR_EXTSRC=5,
	TCR_INTDIS=6,
	TCR_INTREQ=7
};


enum{ADDR_TIMERDATA=0x0008,
	ADDR_TIMERCTRL=0x0009
};

void tmr_init(M68TMR_CTX* ctx){
	ctx->tcr=
		(0<<TCR_INTREQ)|
		(1<<TCR_INTDIS);
	ctx->tdr=0xFF;
	ctx->prescaler=0x7F;
}

bool tmr_exec(M68TMR_CTX* ctx, uint64_t cycles,bool pin){

	uint8_t mode= (ctx->tcr>>TCR_EXTEN)&0x03;
	uint8_t preshift=ctx->tcr&0x07;
	uint64_t count;
	switch (mode){
		case 0:
			count=cycles;
			break;
		default:
			assert(1==2);//not implemented yet
	}

	ctx->prescaler=(ctx->prescaler-count)&0x7F;
	uint8_t n=count>>preshift;
	if (count>ctx->prescaler){
		n+=1;
	}
	bool interrupt= n>ctx->tdr;
	ctx->tdr=(ctx->tdr-n  )&0xFF;

	if (interrupt){
		ctx->tcr|=1<<TCR_INTREQ;
	}

	return interrupt & ( (ctx->tcr& (1<<TCR_INTDIS))==0);
}


void tmr_read(M68TMR_CTX* ctx,uint16_t addr,uint8_t* val){
	switch (addr){
		case ADDR_TIMERDATA:
			*val=ctx->tdr;
			break;
		case ADDR_TIMERCTRL:
			*val=ctx->tcr;
			break;
		default:
			break;
	}
}

void tmr_write(M68TMR_CTX* ctx,uint16_t addr,uint8_t val){
	switch (addr){
		case ADDR_TIMERDATA:
			ctx->tdr=val;
			break;
		case ADDR_TIMERCTRL:
			ctx->tcr=val;
			break;
		default:
			break;
	}
}