ROM Start-up Routines
The first job of the ROM programs is to supervise the start up of the computer. This supervision process can be grouped into three stages:
1.POST (power on self-test):
The POST routine performs a quick reliability. Tests the computer and the ROM programs. This step is very important
To make sure that the computer is ready. These routines check the integrity of the entire system and the status of I/O devices like keyboard, mouse, monitor etc.
Following tasks are performed during this process: .A routine set up a table of address called Interrupt Vector Table (IVT) I In RAM at low end of memory. These addresses are addresses of
Interrupt Service Routines stored inside the ROM-BIOS. Another initialization routine determines what equipment is attached to the computer and than places a record of it at a fixed location in ' memory.
The initialization routine also checks for new equipment. If they found, then they control is transformed to the ROM extension routines, which installs the new equipment and hand over the control back to ROM start-up routines. The initialization routines then continue executing the remaining start up routines.
After the execution of POST routines, initialization routines and ROM
Extension routines, the final part of the start-up procedure is left to the bootstrap loader program. The bootstrap loader program is a short routine that attempts to read disk bootstrap program from the first sector of the disk. After disk bootstrap. program is successfully read into memory, the ROM Bootstrap start-up I procedure ends and the disk bootstrap program takes over the control. The disk bootstrap program subsequently loads 10.SYS from the disk into memory. Then 10.SYS loads MSDOS.SYS into memory, and finally
MSDOS.SYS loads COMMAND.COM into memory. Following this, the DOS prompt (A, B or C) is displayed.
ROM-BIOS stands for Read Only Memory Basic Input/Output System. In the narrowest sense it means a bunch of device control programs, which control I/O operating system.
It is a collection of routines that are part of the hardware of your computer system and is supplied by the computer manufacturer as part of the hardware. Since these routines are permanently recorded and stored in ROM chips, they are often referred to as 'firmware' of the system. The ROM-BIOS is that part of the ROM that is in active use whenever computer is at work. Its job is to provide fundamental services that are needed for the operation of the computer. For the most part, the ROM-BIOS control the computer’s peripheral devices such as display screen, keyboard and disk drives. It consists of BIOS routines that translate a single command, such as tread something from the disk into all steps necessary to actually execute the command.
BIOS work in two directions in a two-sided process, linking the program execution in RAM with the hardware. Let us see how one side of BIOS receives request from programs (either application programs or even DOS) to perform standard ROM-BIOS I/O services. The request made to BIOS gives detailed information as to which device is to be used and for what purpose. The other side of BIOS then communicates with the computer's hardware devices like display screen, disk drives and so on, using detailed command codes fixed for a particular device.
This side of ROM-BIOS also handles any hardware interrupt that a device generates to get attention. For example, whenever a key is pressed, the keyboard generates an interrupt to let the ROM-BIOS know that a key has been hit. (Interrupts are the signals made to the microprocessor asking it to stop whatever it is doing at that movement and start executing the program whose location in the memory is specified when the signals are passed).
ROM BASIC Routines
ROM BASIC acts in two ways. Primarily, it provides the core of the BASIC language, which includes most of the commands and the underlying foundation (such as memory management), those BASIC employees. This disk based BASIC (interpreted basic) relies on ROM BASIC to get much of its work done. So, whenever it is in use it makes use of the ROM BASIC programs. The second role of ROM BASIC comes into play when the booting fails. In such an event, the computer switches control to the ROM BASIC interpreter.
However, these days the PC manufacturers have dispensed with this ROM- BASIC chip (possibly to cut down the cost), therefore if booting fails we don't land into BASIC mode, but get a familiar message Non System disk -Insert system disks and press any key
ROM Extension Routines
Since the original ROM-BIOS could not include support programs for further hardware, ROM Extension are necessary. Non-standard equipment like hard disk, monitor etc., if installed on our system are checked and initialized by ROM extension software. The PC was designed to allow installable extensions to the built in software in the ROM. This additional ROM is usually stored on an adapter card that can be plugged into the empty expansion slots on the motherboard.