MIPS Exceptions Initialization and Handling on Linux

Exceptions and interrupts are un-expected events that disrupt the normal flow of execution. Interrupts are unexpected events from outside the CPU core. Exceptions are from within the CPU, including memory translation exceptions, cache misses, unusual program conditions such as unaligned loads, system calls and traps, and so on.

MIPS architecture adopts precise exception mechanisms, and make it easy for the software to deal with the exceptions. After any exception, CP0 EPC register points to the correct place to restart execution after the exception is dealt with.

Learning the Linux Boot Memory Allocator (MIPS)

Do you ever look at the report of 'cat /proc/meminfo' and wonder why the reported MemTotal is not equal to the amount of memory you have for the system? The followed is an example report from one of my systems. It shows the total memory is 60860kB (59.4MB) from line 2:

1 # cat /proc/meminfo
2 MemTotal:          60860 kB
3 MemFree:           47448 kB
4 Buffers:               0 kB
5 Cached:             7180 kB
6 SwapCached:            0 kB
7 ... 

which is different from the amount of memory that bootloader gave to kernel, 64MB.

1 ...
2 ## Transferring control to Linux (at address 800042f0) ...
3 ## Giving linux memsize in MB, 64 

Step by step to MIPS assembly

In this article, I am going to introduce how to write MIPS assembly. All the examples introduced will able to be assembled or compiled into actual executable programs. They can be executed on a MIPS machine running Linux. So, let's forget the simulator and come to real world :-P. The first is the most simple program.

MIPS calling convention with GCC

In this article, we are going to learn MIPS calling conventions by examples. And we will focus on the one used by GCC. All the examples will be compiled with GCC flag -mno-abicalls not generating SVR4-style position-independent code. Normally -mabicalls should be used, we disable it for the simplification of assembly codes.

Remove allow_branch_to_undefined from U-boot

Old U-boot (v1.1.3 in my example, too old hum?) needs a option -mips_allow_branch_to_undefined for ELDK AS to assemble. We have to modify cpu/mips/start.S so that AS of other toolchains could assemble it without an error - Error: can't resolve `_GLOBAL_OFFSET_TABLE_' {*UND* section} - `L16' {.text section}