POST (Power On Self Test)
The Power On Self Test, or POST, automatically runs every time you turn on your computer. It tests several components of your computer:
The Processor - if the test fails on the CPU, the system stops, usually no error code
ROMs - POST checks the BIOS ROMs, problems usually cause the system to stop with no error code
DMA Controller - if any problems, the system stops
Interrupt Controller - any problems and the system gives an audible error message of one long beep, then a short beep, then the system stops
System timing chip - this chip provides timing signals for the bus and processor, this also gives a long beep and a short beep, then the system stops
Video Card - if this fails, there is a long beep, then two short beeps, then the system halts
RAM - an error here generates a 201: Memory Error message on the screen, any error beginning with 2 is a memory error
Keyboard - an error causes a 301 - Keyboard Failure error message on the screen, followed by a short beep, system may halt or may continue, some systems may ask you to press F1 to continue - which makes no sense
Floppy Drives - any problems cause a 601 - Floppy Disk error
Other devices - POST checks the other ports and parts of the computer, beeps, then continues. It also checks the Master Boot Record (MBR) on the hard drive, if it cannot find the MBR, it will freeze and not continue loading, otherwise it hands control over to the MBR (or DOS Boot Record - DBR - if it is booting from a floppy)
Error codes - the following table summarizes the error codes and what they mean:
Error Code Problem Description
1xx System Board problem
161 CMOS Battery Failure
164 Memory System Size error
2xx Memory related problem
3xx Keyboard problem
4xx Monochrome video problem
5xx Color video problems
6xx Floppy Disk problem
17xx Hard disk problem
BIOS
The BIOS (Basic Input Output System) is the chip that contains low level software for configuring the system's capabilities and communicating with the hardware. The BIOS is usually stored in the ROM and communicates between the software and the hardware so less conflicts present themselves.
Since the BIOS contains the low level software controlling the computer, you may need to replace it in earlier PCs if you have to upgrade the hard drive.
Typical BIOS options include automatic detection of IDE drives, Option to enable/disable the processor/cache, Plug 'N' Play technology, and password protection.
The BIOS writes it configurations to the CMOS (Complementary Metal-Oxide Semiconductor). The CMOS is powered by a small battery so it can retain the settings after the power is turned off. the CMOS is usually not upgradeable.
The CMOS configuration program can usually be started using a key combination when you start up the computer.
When you enter the CMOS configuration program, you can usually change the hard disk type, display type, floppy disk type, boot sequence, serial & parallel configurations, date & time, password, and power management.
Memory
The first type of memory is ROM. ROM is Read-Only Memory which is a form of non-volatile memory, meaning it keeps its information even when there is no power. It is also read only, so you cannot write to it. It contains the setup program and POST (Power On Self Test) software.
RAM, or Random Access Memory, is a volatile type of memory which loses its information when power is not present. RAM is where your software and data is stored when your computer is on.
SRAM is an older type of memory, it stands for Static Random Access Memory. It is an older type of RAM and stores information using transistors.
DRAM, or Dynamic Random Access Information, stores information using capacitors.
PROM, Programmable Read Only Memory, this is a programmable type of ROM. It cannot be erased or changed once it is recorded.
EPROM, Erasable PROM, data can be erased using a UV light on a special window on the chip. This chip can be reprogrammed.
EEPROM, Electronically Erased PROM, can be erased using a special electrical charge. Chips can then be reprogrammed after erased. Most modern day BIOS's are stored in EEPROM.
VRAM is video RAM and is used on video cards.
Cache memory is memory used to cache CPU instructions. L1 is cache memory that is located on the CPU, L2 is cache memory outside of the CPU.
DIP, or Dual Inline Package, is an older type of RAM used mainly in older systems.
RAM
RAM is broken up into DOS memory "sections". The first 640K of memory is Conventional Memory, which is used to load DOS, run programs, and load drivers. DOS is loaded in the first 64K of memory.
Above 640K, but below 1024K is Upper Memory, also called Reserved Memory. 640KB - 768KB is Video Ram, 768KB - 960KB is for BIOS and RAM Buffers, and 960KB - 1024KB is Motherboard BIOS.
Above 1024K is Extended memory. High Memory Area (HMA) is the first 64K of extended memory. Extended memory is mainly used by Windows programs.
Expanded memory is sometimes used by DOS programs and is available up to 32 MB. It is swapped in 16 KB pages (addresses in pages of 16 KB).
In DOS, the MEM.EXE program allows you to determine how memory is being used by DOS. It has several switches, the important ones are highlighted here: /? the help switch, /C the classify switch - gives a report of how memory is used, /D the debug switch - details the first 640KB of memory, /F the free memory switch, shows all free memory blocks in the first 640KB of memory, /M (module) the module switch, shows the starting addresses of the data, program, and how much memory is allocated, and /P the pause switch, displays the output one page at a time.
To use upper memory, you need to make sure the upper memory device driver, EMM386.EXE is invoked in CONFIG.SYS and DOS=UMB is there also.
DOS=HIGH loads DOS into HMA.
Device=HIMEM.SYS is the extended memory device driver.
A Parity Error indicates a problem with RAM. Parity is a simple form of error checking and is used to check RAM.
Ports
Serial Ports
Serial communication is used for modems, mice, and general communication uses. Serial ports are either 25 pin male, "D" Connector, or a 9 pin male, "D" Connector. It sends and receives 1 bit of data at one time.
COM Ports
COM1 and COM3 use IRQ4. COM2 and COM4 use IRQ3. Most PCs only have two external COM ports.
Parallel Ports
Parallel ports are used for printers, scanners, and sometimes drives. It is a 25 pin female, "D" connector. It sends and receives 8 bit of data at one time.
External ports send data in an asynchronous fashion.
Keyboards are either DIN-5 or PS/2. PS/2 is a smaller port and is often on newer computers.
Your mouse is either a 9 pin serial or a PS/2. Again, the PS/2 is smaller and round, while the 9 pin serial is a D-type connector.
Video
Video is a HDA connector. It is a 15 pin female with 3 rows of pins. (VGA & SVGA)
Expansion Slots
The expansion bus allows the computer to be upgraded using different modules. Expansion buses are made up with copper slots where a circuit board can plug into it. The motherboard contains a bus clock which controls how fast information flows to the expansion boards and back.
- The 8-bit expansion bus operates at a maximum of 4.77 MHz (approximately 5), has eight interrupts, four DMA Channels, and one large 62 slot card.
- The ISA (Industry Standard Architecture) bus is a 16-bit bus with a card slightly larger than the 8-bit. The 16-bit cards have an extra piece extending beyond the 8-bit card length. This bus has 16 interrupts and 8 DMA channels. ISA also runs at 8 MHz. ISA buses are also backwards compatible, allowing an 8-bit card to fit into it.
- The MCA (Micro Channel Architecture) bus was a proprietary bus designed by IBM. It is a 16-bit or 32-bit bus and it's clock speed is 10 MHz. It also offered software configuration instead of dip switches and jumper settings.
- EISA (Extended ISA) bus borrowed a lot of features from the MCA bus and expanded on them. It has a 32-bit bus and it has more I/O addresses with no need for interrupts or DMAs. It also still uses the 8 MHz of the ISA card bus to allow for backwards compatibility
- The VESA Local Bus is a local bus type, meaning it is a bus that runs at the same speed as the processor. Typically, its used for video to take advantage of the high speed of transfer to the processor. It is backwards compatible with the ISA, but has an extra slot to make it 32-bits.
- PCI (Peripheral Component Interconnect) Bus is the newest and most advantageous types of all of the buses. It supports both 32-bit and 64-bit data paths to be compatible with 486 and pentium class processors. PCI is also processor independent, allowing it to be on Macintosh, PC, and RISC computers. PCI runs at 33 MHz and has a maximum throughput of 256 megabytes per second.
- PCMCIA (Personal Computer Memory Card Association) is the last type of bus. It is mainly used in laptops and other small computers and in some digital cameras. PCMCIA come in three types, Type I, Type II, and Type III.
Type I cards are 3.3 mm thick. Type II cards are the most common and are 5 mm thick. Type III are mainly used for hard disks and are 10.5 mm thick.
Summary
Below is a chart summarizing the different buses:
Bus Type Bus Size Speed (MHz)
8-bit 8-bit 4.77
ISA 16-bit 8 (10 in turbo mode)
MCA 16-bit or 32-bit 10
EISA 32-bit 8
VESA Local 32-bit Processor Speed
PCI 64-bit Processor Speed
PCMCIA Card 16-bit 33 MHz
Cables & Connectors
Serial Cables
Serial cables communicate between two devices with serial ports. 50 feet is the maximum length a serial cable should be. A null modem cable is used to communicate between two devices (such as computers).
SCSI
SCSI cables communicate between a SCSI port and a SCSI device, or between two SCSI devices. SCSI (Small Computer Systems Interface) is standard on most Macintoshes and optional on most PCs. SCSI is advantageous over other ports because of its high speed data rate and its ability to support up to seven devices (eight devices including the controller card).
Hard drives, tape drives, optical drives, scanners, and CD-ROM drives can all use the SCSI interface. SCSI uses IDs to specify the device, which are numbered from 0-7. Most SCSI host adapters have to be set to SCSI ID 7. Most external SCSI devices use a Centronics 50 or male DB-25 Connectors.
Network Cables
Phone lines (some types of networks) use a jack which resembles a phone jack, a RJ-11 or RJ-12. Another type of network cable, twisted pair, uses RJ-45 connectors. This connector resembles a flat phone jack.
IRQ’s
IRQs (Interrupt Request Lines) and DMAs (Direct Memory Addresses) are an important portion to study as they comprise several questions on the exam. The best method I found was to make flash cards with each IRQ and what it belongs to. Here is a chart of the common IRQs and what they belong to:
IRQ Device Address (Hex) Device
0 Timer 00-0F DMA Controller
1 Keyboard 20-21 Interrupt Controller
2 Wired to IRQ 9 40-43 Timer
3 COM 2 (COM 4) 1F0-1F8 Hard Disk Controller
4 COM 1 (COM 3) 200-20F Joystick Controller
5 Available (often LPT2, sound cards, or network cards) 238-23B Bus Mouse
6 Floppy Disk Controller 278-27F LPT2
7 LPT1 2E8-2EF COM4 Serial Port
8 Clock 2F8-2FF COM2 Serial Port
9 Wired to IRQ 2 300-30F Ethernet Card
10 Unused 330-33F MIDI Port
11 Unused 378-37F LPT1 Port
12 Mouse Port 3E8-3EF COM3 Serial Port
13 Coprocessor 3F0-3F7 Floppy Disk Controller
14 Hard Disk Controller 3F8-3FF COM1 Serial Port
15 Unused
In most PCs, there are 8 DMA Channels. In most modern PCs, DMA shouldn't be used, it just slows it down, but older PCs may use DMA. Channels 4-7 are usually available, while Channel 0 is used to refresh DRAM, Channel 1 is used by a hard disk controller or sound card, and Channel 2 is usually the floppy disk controller.
Another important topic which should be memorized are the common I/O addresses, this table outlines those:
The Processor
The processor, or CPU (Central Processing Unit) controls and directs all activities in the computer. The CPU contains millions of transistors. The CPU is measured by its clock speed in MHz. One megahertz is one million cycles per second. The first IBM PC CPU, the 8088, ran at 4.77 MHz, today's PCs can exceed 700 MHz.
With the chip is a math coprocessor which handles floating point calculations, like algebra and statistics. A math coprocessor, also called an FPU, handles most of the numeric operations.
Most processors have an internal cache which stores frequently used data and instructions. Cache is broken up into two classifications, L1 which is the internal cache and L2 which is the external cache.
Each processor have buses that go along with them. The external bus (system bus) allows the processor to connect with other devices, such as expansion cards and slots. The data bus is used to send and receive data.
The address bus is used to describe memory address locations.
Each CPU handles a different width of each of the different types of buses.
The 8088 chip has a 8-bit data bus and a 20-bit address bus.
The 80286 chip has a 16-bit data bus and a 24-bit address bus.
The 80386SX uses the 80387SX as its coprocessor and has a 32-bit data bus and a 24-bit address bus.
The 80386DX has a 32-bit data bus and a 32-bit address bus.
The 80486SX uses the 80487SX as its coprocessor and has a 32-bit data bus and address bus.
The 80486DX has a 32-bit data and address bus.
The 486DX2 has a 32-bit data and adress bus.
The 486DX4 has a 32-bit data and address bus.
The Pentium has a 32-bit data and address bus.
The Pentium Pro has a 64-bit data bus and a 32-bit address bus.
The Pentium Pro II has a 64-bit data and address bus.
The MMX version of these chips is enhanced for multimedia applications and have several important characteristics. It has 57 instructions for manipulating video, graphic data, and audio. It also has more built-in cache on the chip.
The following tables summarize the processor chips and their characteristics:
CPU Data Bus (bits) Address Bus (bits) Speed (MHz) Transistors
8088 8 20 4.77 29,000
80286 16 24 8-12 134,000
80386SX 32 24 16-20 275,000
80386DX 32 32 16-33 275,000
80486SX 32 32 16-33 1.185 Million
80486DX 32 32 25-50 1.2 Million
486DX2 32 32 33-66 2 Million
486DX4 32 32 75-100 2.5 Million
Pentium 32 32 60-166 3.3 Million
Pentium Pro 64 32 150-200 5.5 Million
Pentium Pro II 64 64 233-400 7.5 Million
CPUs are mounted in different sockets depending on the size of the chip. Also, 486 chips and newer usually contain heat sinks, with fans, which fit on top of the processor chip.
The CPU socket information is summarized in this table:
Socket Voltage Pins Processors Upgrade
0 3.3 168 486DX 486DX2 / 486DX4
1 3.3 169 486DX, 486SX 486DX2 / 486DX4
2 3.3 238 486DX, 486SX, 486DX2 486DX2/4, Pentium
3 3.3 or 5 237 486DX, 486SX, 486DX2, 486DX4 486DX2 / 486DX4
4 5 273 60/66 MHz Pentium Pentium
5 3.3 320 Other Pentium Pentium
6 3.3 235 486DX4 Pentium
7 3.3 321 Other Pentium Pentium
8 3.3 387 Pentium Pro Pentium Pro
There are two types of upgrade methods, ZIF (Zero Insertion Force) and LIF (Low Insertion Force) sockets. ZIF sockets have a mounting bar attached.
