105 lines
3.0 KiB
C
105 lines
3.0 KiB
C
/*
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* z80-boot-rom — a rom-32k variant whose image is a tiny Z80 boot program,
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* for the Phase 0-2 live proof (project/multichip-bus/). Identical to
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* examples/intel/rom-32k.c except for rom_image, which holds:
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*
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* 0000: C3 06 00 JP 0x0006 ; requires reading C3,06,00 over the bus
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* 0003: 00 00 00 NOP x3 ; jumped over
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* 0006: 76 HALT ; Z80 drives HALT low here -> observable
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*
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* Reaching HALT proves the Z80 fetched the multi-byte JP and its target from
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* this ROM across the shared data bus and executed it.
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*/
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#include "velxio-chip.h"
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#include <stdint.h>
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#include <stdbool.h>
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#define ROM_SIZE 0x8000 /* 32 KB */
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static const uint8_t rom_image[ROM_SIZE] = {
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[0x0000] = 0xC3, [0x0001] = 0x06, [0x0002] = 0x00, /* JP 0x0006 */
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[0x0003] = 0x00, [0x0004] = 0x00, [0x0005] = 0x00, /* NOP NOP NOP */
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[0x0006] = 0x76, /* HALT */
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};
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typedef struct {
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vx_pin a[15];
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vx_pin d[8];
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vx_pin ce;
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vx_pin oe;
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vx_pin vcc;
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vx_pin gnd;
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bool driving;
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} chip_t;
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static chip_t G;
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static uint16_t read_addr(void) {
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uint16_t v = 0;
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for (int i = 0; i < 15; i++) if (vx_pin_read(G.a[i])) v |= (1u << i);
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return v;
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}
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static uint8_t image_byte(uint16_t addr) {
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if (addr >= ROM_SIZE) return 0xFF;
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/* honour the programmed low bytes; an erased EPROM reads 0xFF elsewhere */
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if (addr >= 0x10) return 0xFF;
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return rom_image[addr];
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}
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static void drive_data(uint8_t v) {
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for (int i = 0; i < 8; i++) {
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vx_pin_set_mode(G.d[i], VX_OUTPUT);
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vx_pin_write(G.d[i], (v >> i) & 1);
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}
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G.driving = true;
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}
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static void release_data(void) {
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if (!G.driving) return;
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for (int i = 0; i < 8; i++) vx_pin_set_mode(G.d[i], VX_INPUT);
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G.driving = false;
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}
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static void update_outputs(void) {
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int ce_low = (vx_pin_read(G.ce) == 0);
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int oe_low = (vx_pin_read(G.oe) == 0);
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if (ce_low && oe_low) {
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drive_data(image_byte(read_addr()));
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} else {
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release_data();
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}
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}
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static void on_pin_change(void* user_data, vx_pin pin, int value) {
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(void)user_data; (void)pin; (void)value;
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update_outputs();
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}
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void chip_setup(void) {
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char name[4];
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for (int i = 0; i < 15; i++) {
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name[0]='A';
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if (i<10) { name[1]='0'+i; name[2]=0; }
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else { name[1]='1'; name[2]='0'+(i-10); name[3]=0; }
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G.a[i] = vx_pin_register(name, VX_INPUT);
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}
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for (int i = 0; i < 8; i++) {
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name[0]='D'; name[1]='0'+i; name[2]=0;
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G.d[i] = vx_pin_register(name, VX_INPUT);
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}
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G.ce = vx_pin_register("CE", VX_INPUT);
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G.oe = vx_pin_register("OE", VX_INPUT);
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G.vcc = vx_pin_register("VCC", VX_INPUT);
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G.gnd = vx_pin_register("GND", VX_INPUT);
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G.driving = false;
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for (int i = 0; i < 15; i++) {
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vx_pin_watch(G.a[i], VX_EDGE_BOTH, on_pin_change, 0);
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}
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vx_pin_watch(G.ce, VX_EDGE_BOTH, on_pin_change, 0);
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vx_pin_watch(G.oe, VX_EDGE_BOTH, on_pin_change, 0);
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update_outputs();
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}
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