BITS 16
CPU 8086

start:

; sp and all segment registers need to be saved
push bp             ; and we also save bp (??)
push es
mov bp, sp          ; we'll need that to access parameters

xor dx, dx          ; drive 0, head 0
mov es, dx          ; es = 0

mov bx, 0xf000      ; store the read sector there
mov ax, 0x0201      ; read, 1 sector

; params: cylinder, head, sector, out
mov si, [bp+14]
mov ch, [si]
mov si, [bp+12]
mov dh, [si]
mov si, [bp+10]
mov cl, [si]

int 0x13

mov si, [bp+8]
mov [si], ax

pop es
pop bp
retf 8              ; 2x the number of parameters on the stack