Mon, 14 Jan 2013 09:22:12 +0000
More bus error fixes for FreeBee
I have fixed two more bus error handling bugs in FreeBee. First, the CPU core was executing the instruction regardless of whether a bus error occurs when fetching the opcode (which caused it to execute a bogus instruction in such cases). The other one was related to one of my previous fixes - the jump to the bus error vector was at the beginning of the main loop, so it wouldn't be called immediately after the bus error occurred if the timeslot expired, causing the return address to be off.
With these fixes, Unix now runs enough to get into userspace and run the install script (it is also possible to break out and get a shell prompt). However, many commands segfault semi-randomly (or more specifically, it seems that some child processes forked by the shell might be segfaulting before they can exec the command program), so installing the system isn't possible yet. I am not sure exactly what the bug is, but it seems to be related to some function in the shell returning null when the code calling it is assuming that it won't. What the function is, or why it is returning null, I'm not sure (the shell is built without the shared libc and is stripped, making identifying the function harder). I suspect that the function might be in libc, but that is hard to tell.
Author: Andrew Warkentin <andreww591 gmail com>
1 #ifndef SIM__HEADER
2 #define SIM__HEADER
4 unsigned int m68k_read_memory_8(unsigned int address);
5 unsigned int m68k_read_memory_16(unsigned int address);
6 unsigned int m68k_read_memory_32(unsigned int address);
7 void m68k_write_memory_8(unsigned int address, unsigned int value);
8 void m68k_write_memory_16(unsigned int address, unsigned int value);
9 void m68k_write_memory_32(unsigned int address, unsigned int value);
10 void cpu_pulse_reset(void);
11 void cpu_set_fc(unsigned int fc);
12 int cpu_irq_ack(int level);
14 #endif /* SIM__HEADER */