6502-emu/6850.c

#include <stdio.h>
#include <unistd.h>
#include <sys/poll.h>
#include <stdlib.h>

#include "6502.h"
#include "6850.h"

static int n;
static union UartStatusReg uart_SR;
static uint8_t incoming_char;
static int interactive;

void init_uart(int is_interactive) {
	memory[DATA_ADDR] = 0;
	
	uart_SR.byte = 0;
	uart_SR.bits.TDRE = 1; // we are always ready to output data
	
	uart_SR.bits.RDRF = 0;
	incoming_char = 0;
	
	interactive = is_interactive;
}

int stdin_ready() {
	struct pollfd fds;
	fds.fd = 0; // stdin
	fds.events = POLLIN;
	return poll(&fds, 1, 0) == 1; // timeout = 0
}

void step_uart() {
	if (write_addr == &memory[DATA_ADDR]) {
		putchar(memory[DATA_ADDR]);
		if (memory[DATA_ADDR] == '\b') printf(" \b");
		fflush(stdout);
		write_addr = NULL;
	} else if (read_addr == &memory[DATA_ADDR]) {
		uart_SR.bits.RDRF = 0;
		read_addr = NULL;
	}
	
	/* update input register if empty */
	if ((n++ % 100) == 0) { // polling stdin every cycle is performance intensive. This is a bit of a dirty hack.
		if (!uart_SR.bits.RDRF && stdin_ready()) { // the real hardware has no buffer. Remote the RDRF check for more accurate emulation.
			if (read(0, &incoming_char, 1) != 1) {
				printf("Warning: read() returned 0\n");
			}
			if (interactive) {
				if (incoming_char == 0x18) { // CTRL+X
					printf("\r\n");
					exit(0);
				}
				if (incoming_char == 0x7F) { // Backspace
					incoming_char = '\b';
				}
			}
			uart_SR.bits.RDRF = 1;
		}
	}
	
	memory[DATA_ADDR] = incoming_char;
	memory[CTRL_ADDR] = uart_SR.byte;
}