diff options
| -rw-r--r-- | lib/vnf/src/lib.rs | 18 |
1 files changed, 13 insertions, 5 deletions
diff --git a/lib/vnf/src/lib.rs b/lib/vnf/src/lib.rs index 0a3aff1..002c4da 100644 --- a/lib/vnf/src/lib.rs +++ b/lib/vnf/src/lib.rs @@ -899,7 +899,12 @@ impl Machine { return Err("invalid jump!".to_string()); } - let next_address = self.pc + self.current_instruction.size as usize; + // NOTE: as per 6502 specification, the address should be - 1 + // because the 'rts'/'rti' instruction will be the one in charge + // of adding its size to the end PC upon execution. This is kind + // of pedantic but in the end we record the address being pushed + // onto the stack and that should be precise. + let next_address = self.pc + self.current_instruction.size as usize - 1; let low = (next_address as u16 & 0x00FF) as u8; let high = ((next_address as u16 & 0xFF00) >> 8) as u8; @@ -958,12 +963,15 @@ impl Machine { return Ok(()); } - // Pull the previous address from the stack and jump there. Note - // that we have to subtract the current instruction's size - // because it will be re-added after the call to `execute`. + // Pull the previous address from the stack and jump there. + // + // NOTE: as per 6502 specification, the address saved onto the + // stack was the next instruction before the call, but the size + // of the 'rts/rti' should also be accounted. Hence the + 1 to + // the resulting PC. let low = self.pop_stack(false)? as u16; let high = (self.pop_stack(false)? as u16) << 8; - self.pc = (high + low) as usize; + self.pc = (high + low) as usize + 1; self.skip_pc = true; } |