COMPUTER ARCHITECTURE - 1

If you don't have enough time to read those big books by Hayes, Mano and Stallings then proceed. This post does present any contents from the above books. Hope I will present something illustrative on understanding peripheral interface with host system.


The motherboard in a modular PC is divided into a bus board (which has the slots) and a separate processor board.

CPU is the central unit of the board.

PC Architecture

Most of us might not have heard of north bridge and south bridge in our syllabus books.

North bridge and south bridge are combined called core logic chipset. The core logic chipset acts as a switch or router, and routes I/O traffic among the different devices that make up the system.  CPU, the main memory, and the video card need to have faster access across each other. In a modern system, the video card's GPU is functionally a second (or third) CPU, so it needs to share privileged access to main memory with the CPU(s). As a result, these three devices are all clustered together off of the northbridge.

The northbridge is tied to a secondary bridge, the southbridge, which routes traffic from the different I/O devices on the system: the hard drives, USB ports, Ethernet ports, etc. The traffic from these devices is routed through the southbridge to the northbridge and then on to the CPU and/or memory.

The northbridge typically handles communications among the CPU, in some cases RAM, and PCI Express (or AGP) video cards, and the southbridge.

A southbridge chipset handles all of a computer's I/O functions, such as USB, audio, serial, the system BIOS, the ISA bus, the interrupt controller and the IDE channels.

Front side bus acts as a interface between processor and north bridge. Memory bus acts as a interface between Main Memory (RAM) and north bridge. 

Nowadays AGP (Accelerated Graphics Port) is replaced by PCI Express based Graphics card connected to North bridge.

Computers contain some amount of ROM. On a PC, the ROM is called the BIOS (Basic Input/Output System). When the microprocessor starts, it begins executing instructions it finds in the BIOS. The BIOS instructions do things like test the hardware in the machine (POST - Power On Self Test), and then it goes to the hard disk to fetch the boot sector.

Graphics adapter

The above diagram is the block diagram of a graphics adapter. Nowadays Graphics controller chips are repaced by GPUs Graphics Processing unit. The ROM bios with the GPU is something different from ROM bios with CPU in motherboard chipset. The ROM BIOS in GPU works along with ROM BIOS in CPU to select different display modes. The dedicated Video RAM used to store frame buffer.

VRAM has two sets of data output pins, and thus two ports that can be used simultaneously. The first port, the DRAM port, is accessed by the host computer in a manner very similar to traditional DRAM. The second port, the video port, is typically read-only and is dedicated to providing a high throughput, serialized data channel for the graphics chipset.

Memory controller

Memory controllers boards are nothing but host bus adapters in SCSI environment, SD/MMC host controller cards in storage card segment.

Not in Personal storage but in enterprise segment like servers nowadays accelerated memory solutions are carried out to reduce access latency over data base servers in world wide web.

These cards need dedicated processors like one in graphics card, code ROM is different from ROM BIOS.

To use and control the drives effectively, many controllers have their own ROM-BIOS. As for the ROM-BIOS on a graphics adapter, this ROM-BIOS holds several routines for accessing the hard disk controller. The control routines for the floppy drives are already located in the ROM BIOS on the motherboard - do not confuse this ROM-BIOS with the ROM code. The routines in the ROM code control the microprocessor on the controller and cannot be accessed by the CPU on the motherboard, whereas the routines in the ROM-BIOS on the controller support the CPU on the motherboard.

CMOS RAM and Real-time Clock

If the computer is switched off, the PC loses its memory, and therefore doesn’t know what components are installed. At power-up, all drives and components must be initialized. Beginning with the AT, this information was then held by a chip on the motherboard, the CMOS RAM. The feature of this chip is that it needs relatively little power compared with other memory chips. In ATs and all newer IBM-compatibles, a battery or accumulator is present to supply power to this CMOS RAM.


CMOS chip has another function: it includes a real-time clock. When the PC is switched off (or even unplugged) this clock is powered by the battery or accumulator, and is therefore able to update time and data independently. Today you don’t have to provide the time or date at power-up, as the computer reads the CMOS RAM.

Keyboard controller

Serial controller

The central component is a so-called UART (Universal Asynchronous Receiver Transmitter). Unlike the parallel interface, however, the UART doesn’t transfer the data to the peripheral in a parallel way, but converts each byte into a serial stream of individual bits. This stream is transmitted via a single data line, not eight as is the case for the parallel interface. Moreover, the UART adds additional bits, if necessary: start, stop and parity bits. A data packet consisting of multiple data bits and the additional UART control bits is thus formed. 

The number of signal changes per second is called the baud rate. The parity bit serves as a simple validity check for the transmitted data. In this way, much longer distances compared to the parallel interface are possible. 


Serial interfaces in PCs conform to the RS232C standard, which defines the layout and meaning of the connections, and which requires 25 contacts.  Additionally, a reduced version with only nine pins exists.

In the next post we will see why we are going for serial communication.