[CrashCourse CS] #6 Registers and RAM

To store a value and run several operations in a row, we use computer memory.

Making a memory by using loops

• We can create circuits that loop back on themselves.
• To permanently output value 1 : use OR.
  
• To permanently output value 0 : use AND.
  
• Combine them to make AND-OR Latch.
  
• SET input : sets the output to a 1.
• RESET input : sets the out to a 0.
• If SET and RESET are both 0, the circuit outputs whatever was the last.
• This is called a latch - it latches onto a perticular value and stays that way.
• The action of putting data into memory is called writing, whereas getting the data out is called reading.
  
  

Gated Latch

• Having two different wires for input - set and reset - is a bit confusing.
• To make this a little easier to use, we ues a single wire to input data, and another wire that enable writing.
  
• If we put 8 latches side-by-side, we can store 8 bits of information.
• A group of latches operating like this is called a register, which holds a single number, and the number of bits in a register is called its width.
• Early computers had 8 bit registers, then 16, 32, and nowadays have 64 bits wide registers.
  
  

8 bit register

• Putting latches side-by-side takes too many wires for bigger registers.
• Use matrix to save wires.
  
  

16 x 16 256 bit Latch Matrix

• To eanble only a single latch, use AND gate to check if the row and the column wires are both 1.
• Use a read wire to read the data later.
  
  
  

Memory address

• To uniquely specify each intersection, we use memory address.

How to make a memory address

• Making an adrress for row 12 column 8:
  • Since there is a maximum of 16 rows, we store the row address in a 4 bit number.
  • 12 - 1100, 8 - 1000
  • 11001000

Multiplexer

• Multiplexer is a component that selects the latch via address.
  
• Make row of 8 of them to store an 8 bit number (= byte).
• To save an 8 bit number (1 0 0 1 0 1 0 1):
  • Save each bit into a same address for each 256bit block.
  • In total the component can store 256 8bit numbers ( = 256 bytes) at 256 different addresses.

• The way that modern computers scale to megabytes and gigabytes of memory is to keep packaging up little bundles of memory into larger arrangements.
• As the number of memory locations grow, addresses have to grow as well.
  • 8 bits hold enough numbers to provide addresses for 256 bytes of memory
  • To address a gigabyte (billion bytes) - we need 32 bit addreesses.
    
    

Random Access Memory

• We can access any memory location at any time, and in a random order : Random-Access-Memory
  • Computer's memory = RAM
  • Similar to short-term memory for human
• The RAM built here is a SRAM - Static Random-Access Memory (uses latches).
  • Other types : DRAM, Flash Memory, NVRAM...
  • Similar in function so SRAM, but use different circuits to store the individual bits.
• Layers and layers of abstraction...!
  
  

Thoughts

• 아빠가 하는 일이 저런거구나 싶었다.- "ASIC Programmer"
• 메모리도 스위치의 조합으로 만들어진다는게 놀라웠다.