pi.asm

bits 16
org 0

start:
 jmp short entry
 nop
 dw 'PI_10   '             ; disk name
 dw 512                    ; sector size
 db 16                     ; sectors per cluster
 dw 1                      ; reserved sectors
 db 2                      ; number of FAT copies
 dw 512                    ; root entries
 dw 80*2*18                ; total # of sectors
 db 240                    ; media type
 dw 1                      ; sectors per FAT
 dw 18                     ; sectors per track
 dw 2                      ; number of heads
 times 4 dw 0
 db 0, 0                   ; drive number, current head
 db 0x29                   ; signature
 db 0xfd, 0x1a, 0xbe, 0xfa ; id
 db 'PI_10      '          ; volume label
 db 'FAT12   '             ; type

entry:
 ; prepare .com-like environment
 cld
 xor ax, ax             ; clear ax and dx
 cwd
 mov si, 0x100          ; initialize other registers
 mov cx, 0xff
 mov bp, 0x900
 mov di, 0xfffe
 xor sp, sp
 jmp 0x7c0:0x100 - 5
 times start + 0x100 - 5 - $ nop

 push ax
 push cs                ; cs=ds=es=ss for DOS---es not needed really
 pop ds
 push cs
 pop ss

 ; here at 0x100 - based on pi_10.com
 dw 5000
 mov ch,0x7f
 mov es,cx             ; es = 0x7fff
 inc ax
 mov bx,9860
 rep stosw
.loop1:
 mov cx,es             ; cx = 0x7fff
 .loop2:
   imul bp,cx,byte +0x2
   inc bp
   div bp
   xchg ax,dx
    stosw
   xchg ax,dx
   mul cx
   push dx             ; dx:ax += 10 * [es:di]
    xchg ax,bp
    mov ax,10
    mul word [es:di]
    add ax,bp
   pop bp
   adc dx,bp
  loop .loop2
  div word [si]
  add al,0x30
  xchg ax,dx
  stosw
  call prch
  dec bx
 jnz .loop1
 cli
 hlt

prch:
 pusha
 xchg ax,dx
 mov ah, 14
 mov bh, 0
 int 10h
 popa
ret

times start + 0x1fe - $ db 0
db 0x55, 0xaa