Invalid/Valid memory ranges for byte/half/word accesses

[ghost]

I've generated these IO maps by running a full address space scan. The maps themselves should be fairly straightforward, but the format is this:

% $min - $max | error=(0|1)

error=0 means that the range didn't cause an invalid address exception at each address in the range.
error=1 means that the range did cause an invalid address exception at each address in the range.

Some of these ranges are interesting, and I've never seen any documentation of them, for example:

% $fffe0000 - $fffe001f | error=0
% $fffe0100 - $fffe013f | error=0

$fffe0130 is somewhat documented, but nothing else in that range is. I have no clue what any of these do, if they do anything at all.

map-byte.md
map-half.md
map-word.md

The code to generate these maps is roughly (replace read32 and a few 4s for the other sizes):

#include "exception.h"
#include "psx.h"

uint32_t exception_handler_80[4];

extern void as_exn_handler();

int main() {
  // Set up a custom exception handler that just sets $k1 to 1 and returns.

  save_exception_handler(0x80000080, (uint32_t *) exception_handler_80);
  load_exception_handler(0x80000080, (uint32_t *) as_exn_handler);

  // Start scanning the address space

  uint32_t address = 0;
  uint32_t address_of_last_change = 0;
  bool in_error_region = false;

  do {
    asm volatile("li $k1, 0");

    volatile uint32_t data = read32(address);
    bool error_happened;

    asm volatile("move %0, $k1" : "=r"(error_happened));

    if (in_error_region != error_happened) {
      bios_printf("$%08x - $%08x | error=%d\n",
        address_of_last_change,
        address - 4,
        in_error_region);

      address_of_last_change = address;
      in_error_region = error_happened;
    }
  } while (address += 4);

  // Restore previous exception handler

  load_exception_handler(0x80000080, (uint32_t *) exception_handler_80);

  return 0;
}

.set mips1
.set noreorder
.set noat

.text

.global as_exn_handler
.type as_exn_handler, function

as_exn_handler:
    j as_exn_handler_
    nop
    nop
    nop

as_exn_handler_:
    mfc0 $k0, $14
    li   $k1, 1
    nop
    addi $k0, 4
    jr   $k0
    rfe

[rixnobis]

Your scan matches what the page now documents. The unlocked ranges are listed under "Garbage Locations in I/O Area" in https://psx-spx.consoledev.net/unpredictablethings/, FFFE0000h (32 bytes) and FFFE0100h (48 bytes) included, with how reads behave there. The locked gaps around them are in the list just above. FFFE0130h is BCC. Closing.