Loading from Hard Drive to Memory

[mikester2]

I understand how the processor fetches and executes instructions from memory cell by cell, but what i don't get is how do these instructions get loaded from the hard drive and into memory when they are needed. How is the hard drive contacted at the assembly language level, are there special instructios to access it or something ??

Any help would be great.

Cheers

[The Duck]

It's all a matter of IRQ's, I/O addresses, and DMA's... I suggest you do a google search on these topics and read about them.

[rcgreen]

1302
Category: BIOS

INT 13 - DISK - READ SECTOR(S) INTO MEMORY

	AH = 02h
	AL = number of sectors to read (must be nonzero)
	CH = low eight bits of cylinder number
	CL = sector number 1-63 (bits 0-5)
	     high two bits of cylinder (bits 6-7, hard disk only)
	DH = head number
	DL = drive number (bit 7 set for hard disk)
	ES:BX -> data buffer
Return: CF set on error
	    if AH = 11h (corrected ECC error), AL = burst length
	CF clear if successful
	AH = status (see #00234)
	AL = number of sectors transferred (only valid if CF set for some
	      BIOSes)
Notes:	errors on a floppy may be due to the motor failing to spin up quickly
	  enough; the read should be retried at least three times, resetting
	  the disk with AH=00h between attempts
	most BIOSes support "multitrack" reads, where the value in AL
	  exceeds the number of sectors remaining on the track, in which
	  case any additional sectors are read beginning at sector 1 on
	  the following head in the same cylinder; the MSDOS CONFIG.SYS command
	  MULTITRACK (or the Novell DOS DEBLOCK=) can be used to force DOS to
	  split disk accesses which would wrap across a track boundary into two
	  separate calls
	the IBM AT BIOS and many other BIOSes use only the low four bits of
	  DH (head number) since the WD-1003 controller which is the standard
	  AT controller (and the controller that IDE emulates) only supports
	  16 heads
	AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more
	  than 1024 cylinders by placing bits 10 and 11 of the cylinder number
	  into bits 6 and 7 of DH
	under Windows95, a volume must be locked (see INT 21/AX=440Dh/CX=084Bh)
	  in order to perform direct accesses such as INT 13h reads and writes
	all versions of MS-DOS (including v7 [Win95]) have a bug which prevents
	  booting on hard disks with 256 heads, so many modern BIOSes provide
	  mappings with at most 255 heads
SeeAlso: AH=03h,AH=0Ah,AH=06h"V10DISK.SYS",AH=21h"PS/1",AH=42h"IBM"
SeeAlso: INT 21/AX=440Dh/CX=084Bh,INT 4D/AH=02h

http://poli.cs.vsb.cz/misc/rbint/text/1300.html

In 16 bit real mode (like dos), you could use this stuff to get low level access
to drives, reading and writing sectors, or even messing with the bytes between
sectors, changing sector IDs, do a true low level format etc.
Running a modern OS, only the disk driver (deep inside the OS) has
actual intimate knowledge of the disk, so you can access the disk from a
high level programming language.

#include <stdio.h>
#include <stdlib.h>
void main()
{
FILE *myfile;
myfile = fopen("alive.txt","w");
if(!myfile)
{
puts("Some kind of file error!");
exit(0);
}
fprintf(myfile,"I created a file! It's alive!");
fclose(myfile);
}

http://www.dummies.com/WileyCDA/Dumm...e/id-1060.html