To build on the vdi_info post, I am throwing in my oemboot file. I have used this several times over the years in various forms, in general just to get a loader binary into memory. In its current form it uses LBA after doing an int 13 0x41 check error. Are there bugs? Maybe. It works for my purposes.

Here is a brief description of how it works:

  • 1. Scan directory for a loader file with a name defined by LOADFILE, 8.3 format
  • 2. Loads LOADFILE to memory defined by LOAD_SEG and LOAD_OFF, I use 0x7C000.
  • 3. Far jump to start of loader. The jmp far ptr loader_start is around line 470 and before the jump I load registers with “stuff” I want to pass to the loader.

Some simple BPB Error Codes:
1 – No bios extensions – IBM/MS INT 13 Extensions not supported
2 – Root directory read – could not read root directory
3 – LOADFILE not found
5 – FAT error – bad read or sig error

That is about it. The oemboot.asm file is very commented with good info and random thoughts, so I could remember why I had done certain things. The example files are located:

Code repo: Gitlab or My Gitlab

Okay, let’s put my simple test environment together. I make a raw vdi image then run it attached to a VM so I can format it to FAT16 and make the partition active. Next, I use vdi_info.exe to create bootpbp.inc which will be an include for oemboot.asm. I use uasm to compile oemboot.asm with the following commands:

uasm32 -bin [path to includes] -Fo oemboot.bin oemboot.asm

I write the oemboot.bin back to the vdi using the vdi_info.exe -w option. Next, the loader I am using just gets copied to the FAT16 vdi and boot with bochs.

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