kernel/diskio: EBIOS access is first class citizen in kernel

kernel/diskio.asm

@@ -7,6 +7,13 @@ chssiz: equ 8
 
 chstab: resw (4*chssiz)
 
+	; Disk access packet for EBIOS extensions
+dapps:	equ 0 ; byte packet size
+dapnum: equ 2 ; word number of sectors to transfer
+dapbuf: equ 4 ; dword transfer buffer
+dapsec:	equ 8 ; qword absolute sector number
+dapsiz:	equ 16
+
 	section .text
 
 	; Convert between drive number and BIOS dl
@@ -80,30 +87,59 @@ adisk:	push bp
 	push ds
 	push es
 	mov bp, sp
+
+	push ax
+
 	cmp cx, 0xFFFF
-	je .islrg
+	je .lrg
 
-	; build a disk access packet on the stack
-	push ds ; dword transfer buffer
-	push bx
-	push cx ; word number of sectors
-	push cs ; dword starting sector number (CS always 0)
+	; build ebios DAP from registers supplied
+	; qword sector number
+	push cs
+	push cs
+	push cs
 	push dx
+	; dword target buffer
+	push ds
+	push bx
+	; word number of sectors
+	push cx
+	jmp .psz
 
-	; set our freshly created dap as DS:BX
-	mov bx, sp
+.lrg:	; build ebios DAP from int 25h DAP
+	; qword sector number
+	push cs
+	push cs
+	mov ax, [bx+2]
+	push ax
+	mov ax, [bx]
+	push ax
+	; dword target buffer
+	mov ax, [bx+8]
+	push ax
+	mov ax, [bx+6]
+	push ax
+	; word number of sectors
+	mov ax, [bx+4]
+	push ax
+.psz:	; word packet size
+	mov ax, dapsiz
+	push ax
+
+	; DS:SI = SS:SP (ptr to dap)
 	push ss
 	pop ds
+	mov si, sp
 
-.islrg: mov dl, al
-	call fndchs
-	call calchs
+	; set up int 13h subfunction number
+	mov ax, [si+dapsiz]
+	add ah, 0x40
+
+	; get BIOS drive number
 	mov dl, al
-	mov al, [bx+4]
-	mov es, [bx+8]
-	mov bx, [bx+6]
+	call dnconv
+
 	int 0x13
-	jc .ret
 
 .ret:	mov sp, bp
 	pop es