CS255 Sylabus

What can you do with computer memory ???

Available Operations on Main Memory
- You can do exactly 2 things with computer memory:
  1. Transfer one or more bytes FROM memory TO some (other) device (such as the CPU)
  2. Transfer one or more bytes FROM some device (such as the CPU) TO the memory
  The entity that performs the transfer operation is usually the CPU.
Some computer jargons
- The CPU "reads from memory location X" means
  - A copy the value (the bits) stored at memory location (address) X is transfered into the CPU
- The CPU "writes the value V to memory location X" means
  - The value V (in the CPU) is transfered to the memory location (address) X
- CPU can read or write one or more bytes of memory
  (1 byte, 2 bytes, 4 bytes or 8 bytes at a time).
Operational Details of the Memory Read and Write Operation
- The CPU needs to provide the following signals to indicate the desired operation to the memory:
  1. An address to tell memory which data the CPU wants to perform the operation on.
    The Address Bus is used to convey the address information
  2. The desired operation (READ or WRITE).
    There is a special signals on the control bus that is used to indicate whether the CPU wants to perform a READ or a WRITE operation.
- In addition, the value must be transport between the CPU and the memory
  The value is transfered on the Data Bus
The READ Operation
- The following figure shows how the CPU reads the value from the memory location 2453:
  1. CPU sends out the address value 2453 on the address bus
  2. Simultaneously, CPU sends out the signal R/W = 1 on the control bus, which indicates a READ operation
  3. CPU then waits for the data from memory on the data bus
  4. The R/W = 1 signal and the address bus value 2453 will cause the memory to retrieve the value at memory location 2453 to be sent out on the data bus
  5. NOTE: I have used "decimal" values to illustrate the read operation.
    You will soon see that computers do not use "decimal" values, but "binary" values
The WRITE Operation
- The following is an example where the CPU write the value 53 to the memory location 2453:
  1. CPU sends out the address value 2453 on the address bus
  2. Simultaneously, CPU also sends out the value 53 on the data bus
  3. And the signal R/W = 0 on the control bus which indicating a WRITE operation
  4. The R/W = 0 signal along with the address bus value 2453 and data bus value 53 will cause the memory to store the value 53 at the location 2453...
  5. The CPU waits until the memory tells it that the value has been written (with some return signal - not shown in figure)