From c0e1b0efc243377a9b1a12eb6baafb35afdc386b Mon Sep 17 00:00:00 2001
From: mykola2312 <49044616+mykola2312@users.noreply.github.com>
Date: Wed, 7 Aug 2024 17:31:28 +0300
Subject: [PATCH] implement LBA to CHS conversion in assembly

---
 src/mbr_test.s       |  2 +-
 src/test_disk_read.s | 71 ++++++++++++++++++++++++--------------------
 2 files changed, 39 insertions(+), 34 deletions(-)

diff --git a/src/mbr_test.s b/src/mbr_test.s
index 721f8ae..15b1fa2 100644
--- a/src/mbr_test.s
+++ b/src/mbr_test.s
@@ -36,7 +36,7 @@ entry:
 .bootloader:
     call serial_init
 
-    mov $73728, %eax
+    mov $2342424, %eax
     call lba_to_chs
 .halt:
     jmp .halt
diff --git a/src/test_disk_read.s b/src/test_disk_read.s
index 2a9dc73..9244808 100644
--- a/src/test_disk_read.s
+++ b/src/test_disk_read.s
@@ -10,56 +10,61 @@ lba_to_chs:
     # we actually gonna need stack frame for that one lol
     push %bp
     mov %sp, %bp
-    sub $10, %sp
+    sub $12, %sp
     # -4(%bp)   dword   LBA
-    # -6(%bp)   word    cylinder
-    # -8(%bp)   word    head
-    # -10(%bp)  word    sector
-    mov %eax, -4(%bp) # save LBA
+    # -6(%bp)   word    disk_sectors * 2
+    # -8(%bp)   word    cylinder
+    # -10(%bp)  word    head
+    # -12(%bp)  word    sector
+    mov %eax, -4(%bp)       # save LBA
 
-    # chs->head = lba / (conf->num_heads * conf->num_sectors);
-    # (conf->num_heads * conf->num_sectors)
-    movw disk_heads, %ax
-    mulw disk_sectors
-    mov %ax, %bx
-    # lba /
+    # chs->head = (lba % (conf->num_sectors * 2)) / conf->num_sectors;
+    # (conf->num_sectors * 2)
     xor %dx, %dx
-    mov -4(%bp), %eax
-    div %ebx
+    movw disk_sectors, %ax
+    mov $2, %bx
+    mul %bx
+    # ax - num_sectors * 2
+    mov %ax, -6(%bp)
+
+    # (lba %
+    mov %ax, %bx            # disk_sectors * 2
+    mov -4(%bp), %eax       # lba
+    div %ebx                # %
+    # edx, as remainder, now must be divided by num_sectors
+    mov %dx, %ax    # remainder ain't gonna exceed 16 bit value
+    xor %dx, %dx    # always clear god damn EDX it can screw your div/mul!
+    divw disk_sectors
     # ax - head
+    mov %ax, -10(%bp)
+
+    # chs->cylinder = (lba / (conf->num_sectors * 2));
+    mov -4(%bp), %eax       # LBA
+    mov -6(%bp), %bx        # disk_sectors * 2
+    div %ebx                # /
+    # ax - cylinder
     mov %ax, -8(%bp)
 
-    # chs->cylinder = (lba / conf->num_sectors) % conf->num_heads;
-    # (lba / conf->num_sectors)
-    xor %dx, %dx
+    # chs->sector = (lba % conf->num_sectors + 1);
+    mov -4(%bp), %eax       # LBA
     movw disk_sectors, %bx
-    mov -4(%bp), %eax
     div %ebx
-    # % conf->num_heads
-    xor %dx, %dx
-    divw disk_heads
-    # dx, as remainder, now has cylinder
-    mov %dx, -6(%bp)
-
-    # chs->sector = (lba % conf->num_sectors) + 1;
-    xor %dx, %dx
-    mov -4(%bp), %eax
-    divw disk_sectors
-    # dx now has sector num, but we need to increment it
+    # dx - sector, but needs +1
     inc %dx
-    # dx - sector
-    mov %dx, -10(%bp)
+    mov %dx, -12(%bp)
+
+    xor %eax, %eax          # flush eax because it loves to screw up itoa
 
     # now lets print C H S values
-    mov -6(%bp), %ax
+    mov -8(%bp), %ax
     mov $str_cylinder, %si
     call prints_number
 
-    mov -8(%bp), %ax
+    mov -10(%bp), %ax
     mov $str_head, %si
     call prints_number
 
-    mov -10(%bp), %ax
+    mov -12(%bp), %ax
     mov $str_sector, %si
     call prints_number