82 lines
2.9 KiB
C
82 lines
2.9 KiB
C
#include "sgb.h"
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static void command_ready(GB_gameboy_t *gb)
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{
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/* SGB header commands are used to send the contents of the header to the SNES CPU.
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A header command looks like this:
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Command ID: 0b1111xxx1, where xxx is the packet index. (e.g. F1 for [0x104, 0x112), F3 for [0x112, 0x120))
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Checksum: Simple one byte sum for the following content bytes
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0xE content bytes. The last command, FB, is padded with zeros, so information past the header is not sent. */
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if (gb->sgb_command[0] >= 0xF0) {
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uint8_t checksum = 0;
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for (unsigned i = 2; i < 0x10; i++) {
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checksum += gb->sgb_command[i];
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}
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if (checksum != gb->sgb_command[1]) {
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GB_log(gb, "Failed checksum for SGB header command, disabling SGB features\n");
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gb->sgb_disable_commands = true;
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return;
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}
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}
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if (gb->sgb_command[0] == 0xf9) {
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if (gb->sgb_command[0xc] != 3) { // SGB Flag
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GB_log(gb, "SGB flag is not 0x03, disabling SGB features\n");
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gb->sgb_disable_commands = true;
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}
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}
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else if (gb->sgb_command[0] == 0xfb) {
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if (gb->sgb_command[0x3] != 0x33) { // Old licensee code
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GB_log(gb, "Old licensee code is not 0x33, disabling SGB features\n");
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gb->sgb_disable_commands = true;
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}
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}
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}
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void GB_sgb_write(GB_gameboy_t *gb, uint8_t value)
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{
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if (!GB_is_sgb(gb)) return;
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if (gb->sgb_disable_commands) return;
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switch ((value >> 4) & 3 ) {
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case 3:
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gb->sgb_ready_for_pulse = true;
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break;
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case 2: // Zero
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if (!gb->sgb_ready_for_pulse || !gb->sgb_ready_for_write) return;
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if (gb->sgb_command_write_index >= sizeof(gb->sgb_command) * 8) {
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command_ready(gb);
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gb->sgb_ready_for_pulse = false;
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gb->sgb_ready_for_write = false;
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}
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else {
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gb->sgb_command_write_index++;
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gb->sgb_ready_for_pulse = false;
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}
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break;
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case 1: // One
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if (!gb->sgb_ready_for_pulse || !gb->sgb_ready_for_write) return;
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if (gb->sgb_command_write_index >= sizeof(gb->sgb_command) * 8) {
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GB_log(gb, "Corrupt SGB command.\n");
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gb->sgb_ready_for_pulse = false;
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gb->sgb_ready_for_write = false;
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}
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else {
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gb->sgb_command[gb->sgb_command_write_index / 8] |= 1 << (gb->sgb_command_write_index & 7);
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gb->sgb_command_write_index++;
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gb->sgb_ready_for_pulse = false;
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}
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break;
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case 0:
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gb->sgb_ready_for_pulse = false;
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gb->sgb_ready_for_write = true;
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gb->sgb_command_write_index = 0;
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memset(gb->sgb_command, 0, sizeof(gb->sgb_command));
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break;
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default:
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break;
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}
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}
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