From 82bb6371478d8fbaa150d7a37f6b93a1e42b889f Mon Sep 17 00:00:00 2001
From: Nocturn9x <hackhab@gmail.com>
Date: Fri, 18 Nov 2022 17:24:45 +0100
Subject: [PATCH] Segment registers are now zeroed, kernel stack is expanded to
 2MiB, load_disk now does not fill cl automatically

---
 Makefile             |  5 ++++-
 src/boot/mbr.s       | 21 +++++++++++++++------
 src/boot/switch32.s  |  2 +-
 src/boot/util/disk.s |  8 +++-----
 src/boot/util/io.s   |  1 -
 src/entrypoint.s     |  2 +-
 6 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/Makefile b/Makefile
index 03f42ae..9dc16e9 100644
--- a/Makefile
+++ b/Makefile
@@ -88,4 +88,7 @@ image: $(BUILDDIR)/mbr.bin $(BUILDDIR)/kernel.bin
 
 
 run: image
-	qemu-system-x86_64 -drive format=raw,file=$(DISTDIR)/os.img,index=0,media=disk
\ No newline at end of file
+	qemu-system-x86_64 -drive format=raw,file=$(DISTDIR)/os.img,index=0,media=disk
+
+debug: image
+	qemu-system-x86_64 -drive format=raw,file=$(DISTDIR)/os.img,index=0,media=disk -monitor stdio
\ No newline at end of file
diff --git a/src/boot/mbr.s b/src/boot/mbr.s
index 8fde586..de9ebab 100644
--- a/src/boot/mbr.s
+++ b/src/boot/mbr.s
@@ -22,13 +22,21 @@ kernel_offset: equ 0x1000
 ; We save the value of the current boot drive
 mov [boot_drive], dl
 
+; This isn't needed inside the qemu emulator, but
+; real hardware is unlikely to start up with the
+; segment registers zeroed, so we do it here
+mov ax, 0
+mov ds, ax
+mov ss, ax
+mov es, ax
+xor ax, ax
+
 ; Now we setup the stack by setting the
-; base pointer to address 0x9000. The address
-; itself doesn't matter as long as it's far
-; enough away from memory already in use by 
-; the BIOS
-mov bp, 0x9000
-mov sp, bp
+; base pointer to a location that's far
+; enough from where the code for the BIOS
+; is located
+mov sp, 0x9000
+mov bp, sp
 call bios_cls
 mov si, real_mode_msg
 call bios_println
@@ -58,6 +66,7 @@ load_kernel:    ; Loads the kernel into memory
     mov si, loading_kernel_msg
     call bios_println
     mov bx, kernel_offset
+    mov cl, 2
     mov dh, 4
     mov dl, [boot_drive]
     call load_disk
diff --git a/src/boot/switch32.s b/src/boot/switch32.s
index 693a09d..67d8716 100644
--- a/src/boot/switch32.s
+++ b/src/boot/switch32.s
@@ -43,7 +43,7 @@ switch32: ; We're not in 32 bit mode, yay!
     mov fs, ax
     mov gs, ax
 
-    mov ebp, 0x90000 ; We also move the stack further up
+    mov ebp, 0x1f8400 ; We also make the stack larger (2MiB)
     mov esp, ebp
 
     call BEGIN_32BIT ; We call back into mbr.s which loads the kernel
\ No newline at end of file
diff --git a/src/boot/util/disk.s b/src/boot/util/disk.s
index f09be8a..79eb6d3 100644
--- a/src/boot/util/disk.s
+++ b/src/boot/util/disk.s
@@ -17,14 +17,12 @@
 
 
 load_disk:
-    ; Loads 'dh' sectors from drive 'dl' (this
-    ; register is set by the BIOS before calling
-    ; the bootloader) into es:bx
+    ; Loads 'dh' sectors from drive 'dl' starting
+    ; from sector 'cl'
     pusha
     push dx
     mov ah, 0x2   ; Perform a read operation
     mov al, dh    ; Number of sectors
-    mov cl, 0x2   ; 0x1 is us, so 0x2 is the first sector we want
     mov ch, 0x0   ; Cylinder 0
     mov dh, 0x0   ; Head position is 0 too
     int 0x13      ; Once this returns, the data will be in es:bx
@@ -58,4 +56,4 @@ disk_loop:
 
 
 disk_read_error_msg: db "TSOS: Read error: ", 0
-disk_sectors_error_msg: db "TSOS: Sector read count error", 0
+disk_sectors_error_msg: db "TSOS: Incomplete read", 0
diff --git a/src/boot/util/io.s b/src/boot/util/io.s
index 3f8f9ac..96acdcd 100644
--- a/src/boot/util/io.s
+++ b/src/boot/util/io.s
@@ -201,5 +201,4 @@ vga_printh:
         ret
 
 
-NEWLINE: db 0xA, 0xD
 HEX_OUT_LONG: db '0x00000000', 0
\ No newline at end of file
diff --git a/src/entrypoint.s b/src/entrypoint.s
index 96f1a89..a111ed0 100644
--- a/src/entrypoint.s
+++ b/src/entrypoint.s
@@ -17,4 +17,4 @@
 [bits 32]
 [extern kmain]
 call kmain
-jmp $
\ No newline at end of file
+jmp $