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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
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.
http://poli.cs.vsb.cz/misc/rbint/text/1300.htmlCode: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
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.
http://www.dummies.com/WileyCDA/Dumm...e/id-1060.htmlCode:#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);
}
:cool: