From 629fc3e8269577b1579b0b58b6eb4d6ed5143bea Mon Sep 17 00:00:00 2001
From: pantonshire <tom@tomandtally.co.uk>
Date: Sat, 29 Jun 2024 16:45:58 +0100
Subject: [PATCH] get drive geometry

---
 boot0.s      | 60 +++++++++++++++++++++++++++++++---------------------
 boot1.s      | 23 ++++++++++++++++++++
 resources.md |  2 ++
 3 files changed, 61 insertions(+), 24 deletions(-)
 create mode 100644 boot1.s

diff --git a/boot0.s b/boot0.s
index 71f1f90..2e99481 100644
--- a/boot0.s
+++ b/boot0.s
@@ -1,9 +1,17 @@
+; MEMORY LAYOUT
+; 0x0000 - 0x7000: stack
+; 0x7000 - 0x7c00: globals
+;   0x7000 - 0x7001: boot drive number
+;   0x7001 - 0x7002: number of hard disk drives
+;   0x7002 - 0x7003: 
+; 0x7c00 - 0x7e00: boot sector
+
 ; BIOS puts our boot sector at 0000:7c00
 org 0x7c00  
 ; We're (probably) in real mode
 bits 16 
 
-  ; Disable interrupts, fuck knows what an interrupt would do right now
+  ; Disable interrupts
   cli
 
   xor ax, ax
@@ -11,41 +19,45 @@ bits 16
   mov ds, ax
   mov es, ax
 
-  ; Put the stack base at the address of our program text (stack will grow high->low away from it).
-  ; We're not doing anything with a stack right now but who knows we might want to later
+  ; Put stack base at 0x7000. Stack grows high->low, so we'll grow away from our program text at
+  ; 0x7c00. We reserve the range [0x7000,0x7c00) for globals.
   mov ss, ax
-  mov sp, 0x7c00
+  mov sp, 0x7000
 
   ; Segment for VGA (0xb800 * 16 = 0xb8000)
   mov ax, 0xb800
   mov fs, ax
 
+  mov di, 0x7000
+
+  ; Store boot drive number
+  mov ds:[di], dl
+
+  ; Get drive geometry
+  mov di, 0x00
+  mov ah, 0x08
+  int 0x13
+  jb panic
+  mov ds:[di + 1], dl
+
   ; Set VGA mode
   ; https://mendelson.org/wpdos/videomodes.txt
-  mov ah, 0x00
-  mov al, 0x03
+  mov ax, 0x0003
   int 0x10
 
-  ; Reveal the true nature of jen
-  xor bx, bx
-  mov ax, 17
-loop:
-  cmp ax, bx
-  jle done
-  lea si, [womble + bx]
-  mov dh, [si]
-  mov di, bx
-  shl di, 1
-  mov byte fs:[di], dh
-  mov byte fs:[di + 1], 0x1f
-  inc bx
-  jmp loop
-
-done:
+  mov word fs:[0x0000], 0xc048
+  mov word fs:[0x0002], 0xc069
+  mov word fs:[0x0004], 0xc021
   hlt
 
-womble:
-  db "jen is a womble!"
+panic:
+  mov ax, 0x0003
+  int 0x10
+  mov word fs:[0x0000], 0x4f46
+  mov word fs:[0x0002], 0x4f41
+  mov word fs:[0x0004], 0x4f49
+  mov word fs:[0x0006], 0x4f4c
+  hlt
 
   ; TODO: enable A20
 
diff --git a/boot1.s b/boot1.s
new file mode 100644
index 0000000..85d3a2b
--- /dev/null
+++ b/boot1.s
@@ -0,0 +1,23 @@
+org 0x7c00  
+bits 16 
+
+  ; Reveal the true nature of jen
+  xor bx, bx
+  mov ax, 17
+loop:
+  cmp bx, ax
+  jae done
+  lea si, [womble + bx]
+  mov dh, [si]
+  mov di, bx
+  shl di, 1
+  mov byte fs:[di], dh
+  mov byte fs:[di + 0x01], 0x1f
+  inc bx
+  jmp loop
+done:
+  hlt
+
+womble:
+  db "jen is a womble!"
+
diff --git a/resources.md b/resources.md
index fda00db..59bcea9 100644
--- a/resources.md
+++ b/resources.md
@@ -3,6 +3,7 @@
 - <http://www.unixwiz.net/techtips/x86-jumps.html>
 - <https://www.nasm.us/xdoc/2.13.01/nasmdoc.pdf>
 - <https://wiki.osdev.org/Real_Mode>
+- <https://www.eecg.utoronto.ca/~amza/www.mindsec.com/files/x86regs.html>
 
 ## Boot sequence
 - <https://wiki.osdev.org/Boot_Sequence>
@@ -18,4 +19,5 @@
 ## VGA
 - <https://en.wikipedia.org/wiki/Video_Graphics_Array>
 - <https://os.phil-opp.com/vga-text-mode/>
+- <https://mendelson.org/wpdos/videomodes.txt>