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Octocontrabass wrote:Is CS 0?

;;; entry - the entrypoint to the code. Make a short jump past the BPB.
entry:
        jmp short redirect
        nop

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; FAT12 Boot Parameter Block - required by FAT12 filesystem

boot_bpb:
%include "fat-12-data.inc"

redirect:
        jmp boot_base:start


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; start
;;; This is the real beginning of the code. The first order of
;;; business is clearing the interrupts, then setting the
;;; segment registers and the stack pointer.  

start:

;;; constants
boot_base                 equ 0x0000      ; the segment base:offset pair for the
boot_offset               equ 0x7C00      ; boot code entrypoint

Octocontrabass wrote:At this point I'm not sure what else to suggest without debugging it myself. Got a disk image? (I don't have Linux installed anywhere convenient for building it.)

Octocontrabass wrote:It's amazing the things you don't see until you're stepping through it in a debugger.

This does not match this.

Schol-R-LEA wrote:I am pretty sure that using GDB with QEMU would only work for p-mode or long mode code (though I haven't checked).

Schol-R-LEA wrote:The main problem right now seems to be in the routine fat_to_file, but I cannot for the life of me figure out where it is breaking, or why.

Octocontrabass wrote: This magic number is the number of sectors per cluster, right? (Although on a floppy disk, multi-sector reads can be problematic if they happen to cross a track or cylinder boundary...)

Octocontrabass wrote:If part of a file happens to occupy LBA 0xFFFE, you'll stop reading there. (But a floppy disk doesn't have that many sectors.)

Octocontrabass wrote:The cluster number in the FAT is the cluster you want to read.

Octocontrabass wrote:This function clobbers SI, but you didn't document it.

Schol-R-LEA wrote:EDIT: I ended up moving the recent changes to a branch and reverting the master branch to the last semi-working version.

Octocontrabass wrote:You need to preserve BX since it's a parameter to extract_next_fat12_entry.

Octocontrabass wrote:You need to increment the buffer address on each iteration of the loop.

Octocontrabass wrote:Perhaps you wanted CMP/JAE instead of TEST/JZ here? Most clusters will have at least one of the bits in the mask set.

Octocontrabass wrote: (And while I'm at it, the "end of cluster" values are 0xFF8 through 0xFFF; 0xFF0 to 0xFF6 are valid cluster numbers. Of course a floppy disk will not have that many clusters.)

Octocontrabass wrote:This function still clobbers the pointer to its buffer without documenting it, and fat_to_file doesn't preserve the pointer either.