BIOS and Memory
Physical Memory
Objectives:
To discuss BIOS configuration, physical memory, and interrupt request handling
Explain the function of the BIOS
Describe how the BIOS settings are stored
Outline the most common BIOS settings in a PC
Discuss the main types of memory in a PC
Describe how to install a physical memory module on a PC
Describe how interrupt requests work
Outline the most common usage for the various interrupt request lines in a PC
Lesson:
One major type of memory that is used in PCs is called read-only memory (ROM).
ROM is a type of memory that normally can only be read, compared to RAM, which can be both read from and written to.
The values stored in ROM are always there, whether the power is on or not.
A ROM chip can be removed from a PC, stored for an indefinite period of time, and then replaced, and it will still contain the data that was there originally.
ROM cannot be easily modified and for this reason it provides security against accidental
changes to its contents.While in general it is true to say that the contents of ROM cannot be changed, there are several ROM variants with some degree of flexibility in this regard.
Regular ROM is constructed from hard-wired logic and is designed to perform a specific
function.It is highly inflexible and its contents are pretty much permanent.
Programmable ROM (PROM) is a type of ROM that can be programmed using special equipment.
Programming a PROM is called burning and is similar to the way a CD-ROM writer works.
Programs are 'burnt' onto blanks once and they can then be read many times.
Erasable programmable ROM (called EPROM) is a ROM that can be erased and reprogrammed.
A small glass window is installed at the top of the ROM package and the chip that holds the memory is visible through this window.
When ultraviolet light is shone through this window for a specific period of time, the contents are erased and the ROM can be reprogrammed.
This type of ROM is much more useful than regular ROM, but the ultraviolet equipment
that makes it work is quite expensive.Electrically erasable programmable ROM (EEPROM) can be erased and reprogrammed using software.
It is the most flexible of all ROM types.
EEPROM is now frequently used for holding BIOS programs and is commonly known as
Flash memory.Random-access memory (RAM) is used for holding programs and data that is being executed.
It is much faster than ROM but it is very volatile in comparison.
The contents of RAM are lost when the power in a PC is turned off.
RAM needs to be capable of being written to, not just read, because it must hold programs and data that the user is currently working on.
However, the user risks losing data stored in RAM unless it is saved regularly.
Dynamic RAM (DRAM) is a type of RAM that holds its data only if it is continuously accessed by a special logic called a refresh circuit.
Many hundreds of times each second, this circuitry reads and then rewrites the contents
of each memory cell.If this is not done, then the DRAM will lose its contents, even if it continues to have power supplied to it.
This constant refreshing action is the reason the memory is called dynamic.
All PCs use DRAM for their main system memory, because it is cheap and takes up
very little space.The overhead of the refresh circuitry is balanced by the ability to use large amounts of inexpensive, compact memory.
Static RAM (SRAM) is a type of RAM that holds its data without the need for an external refresh for as long as power is supplied to the circuit.
SRAM is faster than DRAM.
However, it is more expensive and takes up much more space than DRAM.
Static RAM is synchronized to the system clock.
This is what makes it much faster than dynamic RAM, which is asynchronous.
In a typical PC, the processor needs to communicate with banks or modules of memory.
A SIMM is a single in-line memory module.
A DIMM is a dual in-line memory module.
Both SIMMs and DIMMs are small circuit boards that hold a group of memory chips and they are placed on the motherboard of a PC.
A SIMM provides a 32-bit-wide path to the actual memory chips, while a DIMM provides a path that is 64 bits wide.
For a processor that requires a 64-bit path to memory - a Pentium, for example - two SIMMs or a single DIMM need to be installed.
DRAM (dynamic random access memory) chips use capacitors that store electrical charges.
The types of DRAM chips include
• EDO RAM
• SDRAM
• SLDRAM
• RDRAM
EDO RAM (Extended Data Out RAM) is a DRAM chip that improves the performance of
FPM (fast page mode) memory.EDO RAM eliminates wait states by keeping the output buffer active until the next cycle begins.
If the memory controller is designed for EDO RAM chips, you can substitute EDO RAM
chips for page mode chips.SDRAM, or Synchronous DRAM, chips are considerably faster than standard DRAM chips.
SDRAM chips are synchronized with the CPU's clock, which eliminates wait states.
SDRAM chips are divided into two cell blocks.
While a bit is being accessed in one cell block, the bit in the other block is being prepared for access, which speeds up data transfer.
SDRAM provides data transfer speeds up to 1.1 GBps.
DDR SDRAM - or Double Data Rate SDRAM - increases this speed up to 2.2 GBps.
SLDRAM (Synchronous Link DRAM) is an enhanced version of SDRAM.
SLDRAM uses a multiplexed bus to transfer data rather than fixed pin settings.
SLDRAM can support data transfer speeds from 1.6 GBps to 3 GBps.
The RDRAM (Rambus DRAM) chip provides a data transfer speed of 600 Mbps.
Concurrent RDRAM chips provide data transfer speed of up to 700 Mbps and are used in video games.
Direct RDRAM chips provide speeds of 1.6 GBps and are used in computers.
Direct RDRAM chips are housed in RIMM modules, which are similar to DIMMs, but have
different pin settings.The table shown here lists the characteristics of various DRAM memory technologies.
Direct RDRAM chips can also be built with dual channels, which increases the data transfer rate to 3.2 GBps.
Display adapters use the following RAM types:
• VRAM
• SGRAM
• WRAM
VRAM - or video RAM -is designed with dual ports so that it can refresh the screen while text and images are drawn into memory.
Video RAM is faster than the standard DRAM used in computers.
SGRAM (Synchronous Graphics RAM) chips are similar to SDRAM chips, but they include enhanced graphic features.
For example, the Block Write and Mask Write functions allow the frame buffer to be cleared faster and selected pixels to be modified faster.
WRAM - or Window RAM - is optimized for display adaptors.
WRAM is faster than VRAM, and like VRAM, it can simultaneously refresh the screen while text and images are drawn into memory.
Let's say you want to install a DIMM or a SIMM on a PC.
The first step is to determine which memory module sockets are to be used.
The best way to do this is to consult the motherboard documentation.
For a new system, it is normal to use the first bank of memory, which normally means the lowest numbered sockets on the motherboard.
Once you have determined the socket, you line up the memory module next to it.
Modules are keyed to prevent incorrect insertion.
The next step is to insert the memory module.
A SIMM must be held at a 60 degree angle to the motherboard and rocked back until it slots in correctly.
A DIMM is pushed gently, but firmly, straight down into the socket.
After insertion, you need to lock the memory module into place.
For a SIMM, this involves tilting the module from a 60 degree to a 90 degree angle, so that it is perpendicular to the motherboard.
If it will not tilt, it may be incorrectly inserted, so you should remove it and reinsert it.
Under no circumstances should excessive pressure be applied.
After you have tilted the module into place, small metal or plastic clips should snap around the module to hold it firmly in place.
For a DIMM, there should be a plastic lever on either side of the socket.
When these levers are tilted upwards, the DIMM is drawn down into the socket and held firmly in place when it can go no further.
Let's see if you can correctly install a DIMM on a motherboard.