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chore: comment open bus impl

This commit is contained in:
Rekai Nyangadzayi Musuka 2022-09-11 07:38:55 -03:00
parent 9f45888910
commit 0027d3f8a3
1 changed files with 14 additions and 4 deletions
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18
src/core/Bus.zig
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@ -116,28 +116,38 @@ pub fn dbgRead(self: *const Self, comptime T: type, address: u32) T {
fn readOpenBus(self: *const Self, comptime T: type, address: u32) T {
const r15 = self.cpu.?.r[15];
const word = if (self.cpu.?.cpsr.t.read()) blk: {
const word = blk: {
// If u32 Open Bus, read recently fetched opcode (PC + 8)
if (!self.cpu.?.cpsr.t.read()) break :blk self.dbgRead(u32, r15 + 4);
const page = @truncate(u8, r15 >> 24);
switch (page) {
// EWRAM, PALRAM, VRAM, and Game ROM (16-bit)
0x02, 0x05, 0x06, 0x08...0x0D => {
// (PC + 4)
const halfword = self.dbgRead(u16, r15 + 2);
break :blk @as(u32, halfword) << 16 | halfword;
},
// BIOS or OAM (32-bit)
0x00, 0x07 => {
// Aligned: (PC + 6) | (PC + 4)
// Unaligned: (PC + 4) | (PC + 2)
const offset: u32 = if (address & 3 == 0b00) 2 else 0;
break :blk @as(u32, self.dbgRead(u16, (r15 + 2) + offset)) << 16 | self.dbgRead(u16, r15 + offset);
break :blk @as(u32, self.dbgRead(u16, r15 + 2 + offset)) << 16 | self.dbgRead(u16, r15 + offset);
},
// IWRAM (16-bit but special)
0x03 => {
// Aligned: (PC + 2) | (PC + 4)
// Unaligned: (PC + 4) | (PC + 2)
const offset: u32 = if (address & 3 == 0b00) 2 else 0;
break :blk @as(u32, self.dbgRead(u16, (r15 + 2) - offset)) << 16 | self.dbgRead(u16, r15 + offset);
break :blk @as(u32, self.dbgRead(u16, r15 + 2 - offset)) << 16 | self.dbgRead(u16, r15 + offset);
},
else => unreachable,
}
} else self.dbgRead(u32, r15 + 4);
};
return @truncate(T, rotr(u32, word, 8 * (address & 3)));
}