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Physical Memory October 2 (Day); September 26 (Night) All computer memory is divided into two categories. Read Only Memory (ROM): o ROM consists of chips that contain programs that are acid-etched into chips at the factory.
o These programs in ROM are often called firmware because they share characteristics
of both software and hardware. o Some ROM is socketed on the motherboard. ROM on the motherboard is responsible for routine operations such as the POST, BIOS, booting, etc. o Expansion cards also have ROM ships that provide programming to instruct the devices. Types of ROM chips: o EEPROM: EEPROM = electrically erasable programmable ROM A.k.a. “Flash ROM” Allow programs to be changed by applying a higher voltage to erase its previous memory before new instructions are written. Characterized by their capacity to be erased and reprogrammed directly on their motherboard or expansion card they are installed on, with no special equipment required. Virtually all motherboards today use EEPROM. o EPROM: EPROM = erasable programmable memory Have a special window that allows the current memory contents to be erased with an intense ultraviolet light, so the chip can be reprogrammed.
o No matter which type of ROM you use, the data stored in a ROM chip is non-volatile
and remains permanently unless intentionally erased or overwritten.
Random Access Memory (RAM): o Remember that programs and data stored on the hard drive needs to be loaded into RAM to enable the CPU to swap data in and out quickly (p. 57). o Remember that the Northbridge acts as the intermediary for all data transfers between RAM and the CPU. For example, the CPU tells the Northbridge to update the data in RAM (p. 57). o RAM is a volatile medium, meaning that it requires continuous electricity to hold data. Losing power for even a split second clears completely (p. 58). o Why Add RAM? Adding or upgrading the RAM on a PC can make a dramatic difference in performance (p. 71). Not only does additional RAM allows you to run more programs at once, but also make your OS more stable and less likely to crash (p. 71).
When you run out of RAM, your computer will begin to use the hard
drive. The process of computer switching from RAM to the hard drive is known as virtual memory (p. 72). o Two Major Categories of RAM (p. 57): Dynamic RAM (DRAM): Memory that must be refreshed or recharged every few milliseconds. In fact, it can only hold a charge for about 4 milliseconds. Static RAM (SRAM): Memory that need not be refreshed. Generally used only by the L1 cache and L2 cache.
o Synchronous vs. Asynchronous:
Synchronous uses a clock signal to manage or synchronize its signals and
run in step with the processor.
Asynchronous means that the CPU waits while the Northbridge fetches
data, usually a period of 10-30 clock cycles.
30-pin SIMM (top); 72-pin SIMM (below) DIMM (p. 67): DIMM = Dual Inline Memory Module DIMMs come in two sizes: 168-pin, 184-pin
168-pin DIMM is 5.4 in. X 1 in.; 184-pin DIMM size is XX x
XX
DIMMs come in different speeds 168-pin DIMM (top); 184-pin DIMM (bottom) Note the different notches along the bottom SO-DIMM (p. 68): SO-DIMM = Small Outline DIMM Used for laptops RIMM (p. 69): RIMM is a package made by the Rambus company. RIMMs are similar size to 184-pin DIMMs, but uses a faster memory bus. RIMM
o Accessing RAM: The CPU turns address wires on and off in certain patterns that represent specific memory addresses. The Northbridge interprets the signal, access the area addressed, and sends the data back (p. 59).
For example, that your computer only had 3 wires on the address bus.
Each wire has to be a 1 or 0. That leaves us with eight (2^3 ) possible addresses (p. 59). 000 100 001 101 010 110 011 111
Today’s address bar has 32 wires meaning that it can address
4294967296 (2^32 ) areas of RAM.
