From 3382e39562bfd04f8c0192164123eb8f8f5ccfc4 Mon Sep 17 00:00:00 2001
From: Joseph Osako <josephosako@gmail.com>
Date: Tue, 23 Aug 2022 23:09:16 -0400
Subject: [PATCH] debugging the ELF loader

---
 src/PC-x86/nasm/fat12/stagetwo.asm | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/PC-x86/nasm/fat12/stagetwo.asm b/src/PC-x86/nasm/fat12/stagetwo.asm
index a59f071..8ec6ea3 100755
--- a/src/PC-x86/nasm/fat12/stagetwo.asm
+++ b/src/PC-x86/nasm/fat12/stagetwo.asm
@@ -240,24 +240,28 @@ find_kernel_code_block:
         call print_decimal_word
         write newline
 
-    .program_header_loop:
+        ; bx = pointer to the first program header
         mov bx, gs:[kernel_raw_offset + ELF32_Header.program_header_table]
-        add bx, kernel_raw_offset
+        add bx, kernel_raw_offset                  ; bx = pointer to the first program header
+    .program_header_loop:
+        ; check to see if the section is loadable
         mov ax, gs:[bx + ELF32_Program_Header.p_type]
         cmp ax, ELF_Header_loadable_type
-        jne .loop_continue
+        jne .loop_continue                         ; not a loadable section, skip
+
+        ; found a loadable section
         ; first, clear the region of memory to load to
         push es
         mov ax, kernel_base
-        mov es, ax
+        mov es, ax                                 ; set es to the segment to later map to higher half
         mov dx, gs:[bx + ELF32_Program_Header.p_memsz]
-        add [section_offset_buffer], dx            ; dx = total size to allocate to the kernel code memory area
         push bx
         memset_rm 0, bx, dx
         pop bx
 
         ; move the code section of the file to the kernel code memory area
-        push ds
+        ; keep ES set to the destination segment
+        push ds                                    ; temporarily set DS = GS so the macro works on the right segments
         mov ax, gs
         mov ds, ax
         memcopy_rm [section_offset_buffer], [bx + ELF32_Program_Header.p_offset], [bx + ELF32_Program_Header.p_filesz]
@@ -267,9 +271,11 @@ find_kernel_code_block:
     .loop_continue:
         ; advance the pointer through the header array
         add bx, ELF32_Program_Header_size
+        add [section_offset_buffer], dx            ; dx = total size to allocate to the kernel code memory area
         loop .program_header_loop
         pop gs
 
+
 load_GDT:
        cli
        call setGdt_rm
@@ -282,7 +288,6 @@ promote_pm:
         mov cr0, eax
 
 
-
         ; Perform far jump to selector 08h (offset into GDT, pointing at a 32bit PM code segment descriptor) 
         ; to load CS with proper PM32 descriptor)
         jmp system_code_selector:PModeMain
@@ -300,13 +305,12 @@ PModeMain:
         mov gs, ax
         mov esp, 0x00090000
 
-
         call init_page_directory
 
         mov eax, cr0
         or eax, Paging           ; set Paging bit in CR0 (Control Register 0)
         mov cr0, eax
-        mov esp, 0xc03fffff
+        mov esp, 0xc03ffffc
 
         ; write 'Kernel started' to text buffer
         write32 kernel_start, 7
@@ -386,7 +390,7 @@ valid_elf_file               db 'Valid x86 ELF32 executable file.', CR, LF, NULL
 
 number_of_sections           db 'Number of sections: ', NULL
 
-section_offset_buffer        resw 1
+section_offset_buffer        dw kcode_offset               ; initialize to the start of the code area
 
 
 %include "init_gdt.inc"