From 6b8f4e8f5124688acc36003d47df28471e8b4837 Mon Sep 17 00:00:00 2001 From: Michel Heily Date: Sun, 18 Oct 2020 09:41:35 -0700 Subject: [PATCH] core: Implement open bus (Reading from unused memory) Former-commit-id: f0223556894a02ad1f85bb81abc18d47b7508445 Former-commit-id: e44b952867197467fa724022a05ba4579b1e2f03 --- core/src/arm7tdmi/cpu.rs | 10 +++++++ core/src/sysbus.rs | 64 ++++++++++++++++++++++++++++++---------- 2 files changed, 59 insertions(+), 15 deletions(-) diff --git a/core/src/arm7tdmi/cpu.rs b/core/src/arm7tdmi/cpu.rs index 8d2c9d4..e933133 100644 --- a/core/src/arm7tdmi/cpu.rs +++ b/core/src/arm7tdmi/cpu.rs @@ -457,6 +457,16 @@ impl Core { self.pc = self.pc.wrapping_add(4) } + #[inline] + pub fn get_decoded_opcode(&self) -> u32 { + self.pipeline[0] + } + + #[inline] + pub fn get_prefetched_opcode(&self) -> u32 { + self.pipeline[1] + } + /// Perform a pipeline step /// If an instruction was executed in this step, return it. pub fn step(&mut self) { diff --git a/core/src/sysbus.rs b/core/src/sysbus.rs index c439fad..870945a 100644 --- a/core/src/sysbus.rs +++ b/core/src/sysbus.rs @@ -263,8 +263,51 @@ impl SysBus { self.scheduler.update(cycles); } + + /// Helper for "open-bus" accesses + /// http://problemkaputt.de/gbatek.htm#gbaunpredictablethings + /// Reading from Unused Memory (00004000-01FFFFFF,10000000-FFFFFFFF) + /// `addr` is considered to be an address of + fn read_invalid(&mut self, addr: Addr) -> u32 { + warn!("invalid read @{:08x}", addr); + use super::arm7tdmi::CpuState; + let value = match self.arm_core.cpsr.state() { + CpuState::ARM => self.arm_core.get_prefetched_opcode(), + CpuState::THUMB => { + // For THUMB code the result consists of two 16bit fragments and depends on the address area + // and alignment where the opcode was stored. + + let decoded = self.arm_core.get_decoded_opcode() & 0xffff; // [$+2] + let prefetched = self.arm_core.get_prefetched_opcode() & 0xffff; // [$+4] + let r15 = self.arm_core.pc; + let mut value = prefetched; + match (r15 >> 24) as usize { + PAGE_BIOS | PAGE_OAM => { + // TODO this is probably wrong, according to GBATEK, we should be using $+6 here but it isn't prefetched yet. + value = value << 16; + value |= decoded; + } + PAGE_IWRAM => { + // OldLO=[$+2], OldHI=[$+2] + if r15 & 3 == 0 { + // LSW = [$+4], MSW = OldHI ;for opcodes at 4-byte aligned locations + value |= decoded << 16; + } else { + // LSW = OldLO, MSW = [$+4] ;for opcodes at non-4-byte aligned locations + value = value << 16; + value |= decoded; + } + } + _ => value |= value << 16, + } + value + } + }; + value >> ((addr & 3) << 3) + } } +/// Todo - implement bound checks for EWRAM/IWRAM impl Bus for SysBus { #[inline] fn read_32(&mut self, addr: Addr) -> u32 { @@ -273,7 +316,7 @@ impl Bus for SysBus { if addr <= 0x3ffc { self.bios.read_32(addr) } else { - 0 // TODO open-bus + self.read_invalid(addr) } } EWRAM_ADDR => self.ewram.read_32(addr & 0x3_fffc), @@ -292,10 +335,7 @@ impl Bus for SysBus { } GAMEPAK_WS2_HI => self.cartridge.read_32(addr), SRAM_LO | SRAM_HI => self.cartridge.read_32(addr), - _ => { - // TODO open-bus - 0 - } + _ => self.read_invalid(addr), } } @@ -306,7 +346,7 @@ impl Bus for SysBus { if addr <= 0x3ffe { self.bios.read_16(addr) } else { - 0 // TODO open-bus + self.read_invalid(addr) as u16 } } EWRAM_ADDR => self.ewram.read_16(addr & 0x3_fffe), @@ -325,10 +365,7 @@ impl Bus for SysBus { } GAMEPAK_WS2_HI => self.cartridge.read_16(addr), SRAM_LO | SRAM_HI => self.cartridge.read_16(addr), - _ => { - // TODO open-bus - 0 - } + _ => self.read_invalid(addr) as u16, } } @@ -339,7 +376,7 @@ impl Bus for SysBus { if addr <= 0x3fff { self.bios.read_8(addr) } else { - 0 // TODO open-bus + self.read_invalid(addr) as u8 } } EWRAM_ADDR => self.ewram.read_8(addr & 0x3_ffff), @@ -358,10 +395,7 @@ impl Bus for SysBus { } GAMEPAK_WS2_HI => self.cartridge.read_8(addr), SRAM_LO | SRAM_HI => self.cartridge.read_8(addr), - _ => { - // TODO open-bus - 0 - } + _ => self.read_invalid(addr) as u8, } }