Blast Corps (U) (V1.1) ROM Extending
Updated: 2016 Feb 13
By: queueRAM
For updates, check: http://origami64.net/showthread.php?tid=334&pid=2566

This doc describes the process used to extend the Blast Corps (U) (V1.1) ROM 
to facilitate hacking and modifying levels. The vanilla ROM isn't suitable 
for modifying because the offsets of the level and vehicle data are stored in 
code and data blocks that are gzip compressed. Changing these offsets would 
involve inflating the gzip code, changing the offsets in code, and compressing 
as gzip and hoping the new file size isn't larger than the current one.

A better approach is to store the level offsets in the ROM and modify the 
lookup code to reference this new table.  This also has the benefit of 
reducing the code size.  Below are the steps needed to extend the Blast 
Corps (U) (V1.1) ROM.

1. Store Level Offsets at 0x7FA000 in ROM
There are four offsets stored for each level
 a. level start
 b. level end
 c. display list start
 d. display list end
The game code only needs a. and d. since both blocks are copied together, 
however, having all four makes it quicker for the editor to look things up.
0x7FA000 is somewhat arbitrarily chosen as the first address with three zeros 
in the unused block from 0x7FA1E0-0x7FFFFF.

2. Inflate the hd_code_text.raw and hd_code_data.raw and level gzip blocks:
787FD0-7D73B4  hd_code_text.raw  main asm code
7D73B4-7E3AD0  hd_code_data.raw  data for main asm
4ACC10-4B71AB  chimp.raw         Simian Acres
4B71AB-4B8960  chimp_dl.raw      Simian Acres display list

hd_code_data.raw.gz is not modified, but it needs to be reinserted immediately 
after hd_code_text.raw.gz for the inflate methods to work correctly.

3. Patch code at 0x01199C in hd_code_text.raw
This is done with bass MIPS assembler:
> bass -o hd_code_text.raw hd_code_text.asm

Description of code changes:
proc_8025615C: # begin 8025615C (01199C), T6 = Level ID
  // existing code
  addiu $sp, $sp, -0x28
  sw    $a0, 0x28($sp)
  lw    $t6, 0x28($sp)
  sw    $ra, 0x1c($sp)
  sw    $a1, 0x2c($sp)
  sltiu $at, $t6, 0x3c
  beqz  $at, .Lproc_8025615C_8A4
  sw    $a2, 0x30($sp)
  // new code starting at 119BC/8025617C
  sll   t6, t6, 0x4    // each entry is four words (* 16)
  lui   at, 0xB080     // %hi(0xB07FA000), 0xB0000000 = ROM
  addu  at, at, t6     //
  lw    t6, 0xA000(at) // %lo(0xB07FA000)
  sw    t6, 0x24(sp)   //
  lw    t7, 0xA00C(at) // %lo(0xB07FA00C)
  sub   t7, t7, t6     // compute length
  lw    t4, 0x30(sp)   //
  sw    t7, 0x0(t4)    //
  b     0x12240        // skip over old code
  
  // replace this with nops

.Lproc_8025615C_8A4: // offset 0x12240
  addiu $t7, $zero, 0xa
  addiu $t8, $zero, 1
  sw    $t8, 0x14($sp)
  sw    $t7, 0x10($sp)
  lw    $a0, 0x24($sp)
  lw    $a1, 0x2c($sp)
  lw    $a2, 0x30($sp)
  jal   proc_8028B4C4
  addiu $a3, $zero, 0xc
  lw    $ra, 0x1c($sp)
  addiu $sp, $sp, 0x28
  jr    $ra
  nop

4. Compress patched hd_code_text.raw in gzip:
> gzip -c hd_code_text.raw > hd_code_text.raw.gz

5. Insert the hd_code_text.raw.gz and hd_code_data.raw.gz back-to-back in the 
   Blast Corps ROM at offset 787FD0 and Simian Acres level at 0x0x800000. 
   Also update the Simian Acres offset in the level lookup table.  This is 
   done with bass MIPS assembler bass.
> bass -o "Blast Corps (U) (V1.1).z64" blast_corps_rom.asm