From 3ffab6de1f4ecba15c5dbc8c645146662d83b64f Mon Sep 17 00:00:00 2001
From: Ry <ryanst755@gmail.com>
Date: Sat, 5 Nov 2022 22:57:48 -0700
Subject: [PATCH] Initial work on disk reading

---
 src/disk.s   | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/kernel.s |  4 +++
 2 files changed, 81 insertions(+)
 create mode 100644 src/disk.s

diff --git a/src/disk.s b/src/disk.s
new file mode 100644
index 0000000..b4f204f
--- /dev/null
+++ b/src/disk.s
@@ -0,0 +1,77 @@
+SECTOR_SEG  equ 0x0000
+SECTOR_ADDR equ 0x7C00
+
+; get and store drive parameters
+; inputs:
+; 	none
+; outputs:
+; 	none
+disk_init:
+	push es
+	xor ax, ax
+	mov es, ax ; ES:DI should be zero to prevent some BIOS bugs apparently
+	mov di, ax
+	mov ah, 0x08
+	mov dl, byte [bootdisk]
+	int 0x13
+	inc dh
+	movzx bx, dh
+	mov word [bootdisk_heads], bx
+	and cx, 63
+	mov word [bootdisk_sectorspt], cx
+	pop es
+	ret
+
+; read a sector into the sector buffer
+; inputs:
+; 	SI: LBA
+; outputs:
+; 	none
+read_sector:
+	push ax
+	push bx
+	push es
+.retry:
+	mov es, SECTOR_SEG
+	mov bx, SECTOR_ADDR
+	call lba2chs
+	mov ax 0x0201
+	int 0x13
+	jc .retry
+	pop es
+	pop bx
+	pop ax
+	ret
+
+; convert LBA addressing to CHS addressing
+; these outputs can be fed directly into int 0x13, AH 0x02
+; based on this StackOverflow answer:
+; https://stackoverflow.com/questions/45434899/why-isnt-my-root-directory-being-loaded-fat12/45495410#45495410
+; sector:   (LBA mod SPT) + 1
+; head:     (LBA / SPT) mod heads
+; cylinder: (LBA / SPT) / heads
+; inputs:
+; 	SI: LBA
+; outputs:
+; 	DH: head
+; 	CH: cylinder
+; 	CL: sector/cylinder
+lba2chs:
+    push ax
+    mov ax, si
+    xor dx, dx
+    div word [bootdisk_sectorspt] ; LBA / SPT
+    mov cl, dl
+    inc cl                        ; CL = (LBA mod SPT) + 1
+    xor dx, dx
+    div word [bootdisk_heads]     ; (LBA / SPT) / heads
+    mov dh, dl                    ; DH = (LBA / SPT) mod heads
+    mov ch, al                    ; CH = (LBA / SPT) / heads
+    shl ah, 6                     ; store upper 2 bits of 10-bit cylinder into...
+    or cl, ah                     ; ...upper 2 bits of cector (CL)
+    pop ax
+    ret
+
+bootdisk: db 0
+bootdisk_heads: dw 0
+bootdisk_sectorspt: dw 0
diff --git a/src/kernel.s b/src/kernel.s
index cb59762..8235ca1 100644
--- a/src/kernel.s
+++ b/src/kernel.s
@@ -7,6 +7,9 @@ kjmp: ; MUST be a jmp due to loader config
 %include "int21/int21.s"
 
 kernel_entry:
+	mov byte [bootdisk], dl
+	call disk_init
+
 	mov ah, 0x9
 	mov dx, hello_string
 	int 0x21
@@ -15,4 +18,5 @@ kernel_entry:
 
 hello_string: db "hello world!", 13, 10, '$'
 
+%include "disk.s"
 %include "vga.s"