Memory

THIS WEEK: Memory, encoders and stacks

• Memory – different types
• Need encoders/decoders for interrupt handling -
• 4 to 2 priority encoder used to detect hardware interrupts
• 2 to 4 used for multiplexing and memory addressing.
• Stacks look like n-dimensional shift registers

Building a computer? Things we need:

Fast memory (RAM) Persistent

• program and data memory encoders (Disks) Stack

Central processing Motherboard unit (CPU) registers IP

Screen ALU Keyboard

Input / Output accel

4 Channels cards Mouse

ROM (read only memory): All reading at full speed. Just get the address and go there.

• contents built-in at time of manufacture.
• PROM – Programmable read only memory. Programmed by a once-only irreversible operation, e.g. in factory.
• EPROM – Erasable Programmable read only memory. Can be removed from the computer and erased and programmed (slowly) by using special apparatus (e.g. UV light).
• Sometimes called “field-programmable”, i.e. “in the field”
• Bulk erased: every byte erased at the same time
• Byte programmable: write bytes one by one.
• EEPROM – Electrically erasable read only memory. Can be erased and reprogrammed byte by byte in situ, but writing is slower than normal reading.

RAM random access memory.

• A misnomer it should be RWM (read write memory) – both ROM and RAM allow random access.
• Static RAM – retains information until power removed. Fast, larger area of silicon per byte, modest power requirement.
• Dynamic RAM – retains information as long as the contents are refreshed frequently enough. Smaller area of silicon per byte, low power requirement.
• Does not use flip-flops.
• Uses tiny capacitors to store electric charge.
• Because the charge leaks away have to rewrite (“refresh”) every few milliseconds.
• SDRAM (Synchronous DRAM)
• Hybrid of dynamic and static technology
• “clocked” by the main CPU clock
• DDR (double data rate) SDRAM
• Chips produce data on rising and falling edges of the clock
• Higher data rates, eg 64 bits / nsec
• FRAM – Ferroelectric RAM
• Uses atom position in unit cell (in theory). Actual density about the same as DRAM. Non-volatile (no power / refresh to keep state).
• Used in specialist devices (where you never want to "turn it off and on again" to fix it). Random access
• Flash memory – (EEPROM)
• Charge stored between insulators. Write bit by injecting electrons through a barrier layer (physically damaging it). Used in USB drives. Good for about 30,000 writes. Random access
• Core memory
• Magnetic "core" memory: each bit is stored as the direction of magnetisation of tiny rings (like doughnuts). 8 1955-1975. 1000 x slower than DRAM.

MEMORY ADDRESSING A a byte is typically the smallest addressable

• RAM consists of one or many chips chunk of memory. We
• RAM is organised into words (of e.g. 32 bits) can address the 4 bytes
• Words are grouped into pages. in a 32-bit word, but we
• Words are selected by an address can't address individual
• A number of m bits bits in each byte.
• Control bits specify whether to read or write Exceptions include
• Bits stored in the selected word go to or from the data busMicron 3D-XPoint

Address Data bus

(m bits) (n bits)

C R W

EXAMPLE

• Consider a Computer with 1024 Mbyte (1GB) RAM
• How many bits needed to address all bytes? Hint: 1024 = 210 and 1,000,000 ~= 220
• Alternative to normal RAM addressing:
• Hardware stack
• Many different types of memory:
• ROMS variants
• RAM variants
• Trade offs of speed, space, expense, longevity
• All slower to access than registers
• Memory addressing:
• bits need to address each individual byte (via chip, page, word).
• Alternative to normal RAM addressing:
• Hardware stack