From fde87d651c2fbad48dcd27b91a38814746f856e2 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 15 Aug 2020 21:15:31 +0000
Subject: [PATCH] kernel: read A: geometry from BIOS

---
 boot/kernel.asm  |  8 +++---
 kernel/drive.asm | 71 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+), 4 deletions(-)
 create mode 100644 kernel/drive.asm

diff --git a/boot/kernel.asm b/boot/kernel.asm
index 3641b8e..bf7b869 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -48,10 +48,7 @@ intlp:		movsw
 		mov bx, 0
 		call fd_write
 
-		call dnconv
-		mov al, dl
-		call select
-		call load_bpb
+		call drives_init
 
 loop:		int 0x28
 		jmp loop
@@ -241,9 +238,12 @@ fputc:	push ax
 	%include "kernel/fd.asm"
 	%include "kernel/con.asm"
 
+	%include "kernel/drive.asm"
 	%include "kernel/diskio.asm"
 	%include "kernel/fat.asm"
 
+zero:	dw 0
+
 section .data
 
 vects:	dw int20h, int21h, retf, retf
diff --git a/kernel/drive.asm b/kernel/drive.asm
new file mode 100644
index 0000000..3712f51
--- /dev/null
+++ b/kernel/drive.asm
@@ -0,0 +1,71 @@
+section .bss
+
+drives:		equ 16
+
+; drive table entry
+struc drive
+.biosnum:	resb 1
+.flag:		resb 1
+.spc:		resw 1
+.cylinders:	resw 1
+.dpt:		resb dpt_size
+endstruc
+
+		; ptr into currently selected drive
+drive_ptr:	resw 1
+
+drive_table:	resb (drives * drive_size)
+
+section .text
+
+drives_init:	int 0x11
+		mov cl, 6
+		shr ax, cl
+		inc ax
+		and ax, 3
+		mov cx, ax
+
+		mov bx, drive_table
+		xor dx, dx
+
+		mov [bx+drive.biosnum], dl
+		; defaults if 13h,AH=8 fails (XT 360kb)
+		les di, [0x1e * 4]
+		mov cx, 0x2709 ; 40 cylinders, 9 sectors
+		mov dh, 1 ; 2 heads
+		; ask bios
+		push bx
+		mov ah, 8
+		int 0x13
+		pop bx
+		; copy dpt
+		push es
+		push ds
+		pop es
+		pop ds
+		push cx
+		mov si, di
+		lea di, [bx+drive.dpt]
+		mov cx, 11
+		rep movsb
+		pop cx
+		; restore ds
+		push cs
+		pop ds
+		; get and save number of cylinders
+		mov ax, cx
+		xchg al, ah
+		rol ah, 1
+		rol ah, 1
+		and ax, 0x03FF
+		inc ax
+		mov [bx+drive.cylinders], ax
+		; save spt
+		mov ax, cx
+		and al, 0x3F
+		mov [bx+drive.dpt+dpt.lastsector], al
+		; multiply with heads and save sectors per cylinder
+		inc dh
+		mul dh ; ax = al * dh
+		mov [bx+drive.spc], ax
+		ret