Main Memory

A larger than Register File grid of slower memory cells Stores most info to be processed by the CPU (Almost like a 2D array)

Problem: We can’t store 1 billion registers with a register file!

• I.e a phone can store a billion 32-bit numbers!
• Too many wires and flip flops; each flip flop has too many gates

RAM! (Main Memory)

• Main Memory is a collection of addressable memory units
  • Each cell is in units of bytes
  • bytes = 8 bits
  • A word is 4 “sections in memory” or 4 bytes.
  • For us, a word is 4 bytes / 32 bits
• Like a register file
• You have at most 2^m rows where m is the bit-length of the address you’re passing.
• Each row is n bits.
  • So the size of memory is 2^m * n bits in total
• Different from a register file cuz instead of an array of registers, we have rows of memory units / storage cells
• DRAM cells are for big memory cuz they are so tiny
  • SRAM is fast tho

Organization

• For this example assume n=3 even though it’s not very practical. Usually it’s n=8(a byte)
  • Usually 4 of the 8 bit rows are accessed as words as we defined above
• Anyway the wordline determines which row (a byte, 8 bits, in MIPS, that we we are reading from)
• The bitline is attached to the cells of the wordlines and sends them to their output all nicely.
• Cells by default output 0 when the wordline connected is inactive (refer to the diagrams above), but if a wordline is active and a cell outputs 1 into the bitline connected already to 0, we get a short circuit.
  • Solution? Data Bus
  • If we want to read from the cell, we pull, from the bits into the bitline, through a tri-state buffer
    • If reading, we set EN for the buffer to true such that everything else is disconnected = no short circuit!