RAM、ROMおよびメモリの種類

Memory woven by hand

• The Apollo Guidance Computer that flew to the Moon kept its programs in core rope memory: wires threaded by hand through or around tiny magnetic rings, one pass for a 1, the other for a 0.
• Once woven, nothing could be changed. It was read-only memory in its purest form, and it meant the flight software had to be finished months before launch.
• Its working data lived in a separate, erasable memory that lost everything when the power went.
• Every computer since has the same two kinds of main memory. This lesson is about telling them apart and choosing between them.

RAM and ROM

• RAM 随机存取存储器 (Random Access Memory) is volatile 易失性: its contents are lost when the power is off. It holds the operating system, the running programs and their data, and it is read and written constantly.
• ROM 只读存储器 (Read-Only Memory) is non-volatile 非易失性: it keeps its contents without power. It is written once, and in normal use it is only read.
• ROM holds the firmware 固件 and bootstrap 引导程序 that start the computer, or the whole program of an embedded system. RAM is large and can usually be increased; ROM is small and fixed.

RAM is volatile and read/write; ROM is non-volatile and read-only

Worked example: two favourite questions

The start-up sequence

• Power on. The processor reads its first instructions from ROM, because RAM is still empty.
• The firmware checks the hardware, then loads the operating system from secondary storage into RAM.
• From then on the processor works from RAM. ROM starts the system; RAM holds the active work.

A RAM module: fast, volatile, and empty until the firmware fills it

SRAM and DRAM

• SRAM 静态RAM (static RAM) stores each bit in a flip-flop 触发器 of several transistors. It is fast and uses little power when idle, but it is expensive and holds fewer bits per chip.
• DRAM 动态RAM (dynamic RAM) stores each bit as charge on one tiny capacitor 电容器 with one transistor. It is cheaper and denser, but slower, and the charge leaks away, so every cell must be refreshed 刷新 thousands of times a second.
• So SRAM is used for the processor's small, fast cache 高速缓存, and DRAM for the large main memory.

Worked example: why this memory here?

PROM, EPROM and EEPROM

• PROM 可编程只读存储器 (Programmable ROM) is written once by the user with a programmer, by burning fuses. It cannot be erased.
• EPROM 可擦可编程只读存储器 (Erasable Programmable ROM) is erased by strong ultraviolet light through a quartz window, the whole chip at once, and it must be taken out of the circuit to do it.
• EEPROM 电可擦可编程只读存储器 (Electrically Erasable Programmable ROM) is erased and rewritten electrically, a byte at a time, while it stays in the circuit. Flash memory is a derivative optimised for erasing whole blocks quickly.

Worked example: "give two differences"

Choosing the memory for the job

• Processor cache: SRAM, because it must be fastest and can be small.
• Main memory of a PC, phone or embedded system: DRAM, because it must be large and cheap.
• Firmware that may need updating in the field: EEPROM or flash, so a fix can be installed without opening the device.
• Firmware that must never change, made in millions: mask ROM or PROM, cheapest and safest.

Marks that slip away

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