Readme

This is a test to get a PIC32MZ2048ECG064 running without the MPLAB IDE. There is an Assembly file and a Makefile. The generated HEX can be uploaded using the MPLAB IPE. Generating C code is fairly similar, but uses gcc instead of as.

Links

About the chip: https://www.microchip.com/wwwproducts/en/PIC32MZ2048ECG064
About the architecture: https://www.cs.cornell.edu/courses/cs3410/2008fa/MIPS_Vol1.pdf
About the instruction set: https://www.cs.cornell.edu/courses/cs3410/2008fa/MIPS_Vol2.pdf

Notes to self

Instruction set

This chip has a somewhat larger instruction set than MIPS32; it is a MIPS32 microAptiv. For this extension, see §50 of the PIC32 reference manual (to be found on the chip information page linked above). It is not clear to me how widespread this instructions set is (i.e. whether it would make sense to rely on it being available for code generation or so).

Branch delay slots

Watch out for the branch/jump instructions. The following instruction is always executed, regardless of whether the branch is taken or not. This instruction is said to be in the 'branch delay slot'. For example, this does not loop endlessly even though the body of main_loop_delay seems empty:

    lui $1,0xf
main_loop_delay:
    bne $1,$0,main_loop_delay
    addiu   $1,-1

Also note that when the file ends with a branch/jump, a nop must follow to avoid unpredictable behaviour (the following instruction could be anything otherwise).

lw and lui

One needs lui to load an address, e.g. to clear TRISB:

    lui $2,%hi(TRISB)
    sw  $0,%lo(TRISB)($2)

Using lw instead of lui is an easy mistake, assembles fine, but you usually end up reading out of memory leading to a reset.