Modify GP32's MMU tables and change caching settings

@sang

Published in 

GP32

 · 8 months ago

The following simple example shows how to modify GP32's MMU tables in order to change caching settings. This is especially useful if you want to move your frame buffers into "user RAM area" i.e. somewhere between 0x0C000000 and 0x0C79C000. The example uses BIOS call to do the job. Keep in mind that the granularity of MMU pages is 4KB, which means that the addresses you feed into the BIOS call must 4KB aligned addresses.

    // C example and using SDK - libgpstdlib.a 
    // 

    void swi_mmu_change( void* start_addr, void* end_addr, ulong flags ); 

      start_addr - a starting address of the memory region you 
                   want these MMU changes to affect. 
      end_addr   - an end address of the memory reqion you want 
                   these MMU changes to affect. 
      flags      - This value is ORed to the second level MMU page 
                   table descriptor. Only the low 12 bits are used. 
                   use: 
                     0xFF2 - to disable cache and writeback 
                     0xFFA - to enable cache and disable writeback 
                     0xFFE - to enable cache and writeback 

    For example to turn on caching but disabling writeback from 
    0xc66A000 to 0xc700000 call BIOS function with the following 
    parameters: 

      swi_mmu_change( 0xC66A000, 0xC6FFFFF, 0xFFA ); 

    @ ASM example and not using SDK 
    @ 

    r0 = start_addr 
    r1 = end_addr 
    r2 = flags 

    BIOS call: SWI #0x02 

    For example to turn on caching but disabling writeback from 
    0xc66A000 to 0xc700000 call BIOS function with the following 
    parameters: 

      ldr r0,strt 
      ldr r1,end 
      ldr r2,flg 
      swi #0x02 
      ... 

      strt: .word 0xC66A000 
      end:  .word 0xC6FFFFF 
      flg:  .word 0xFFA

So when you are defining caching for your custom frame buffers make sure that the writeback is disabled. The BIOS disables caching and writeback for frame buffers but I have tested frame buffers with cache on and they seemed to work (no guarantees though). You can play safe and use 0xFF2.