Flip-flops and Registers

Flip-flops

Store 1 bit - limitation as computers usually use 32 or 64 bit numbers

Synchronous digital circuits

Registers

Comprised of flip-flops

Store data being used in CPU

Fastest type of storage

Usually large enough to hold a word

-ve expensive to make so use RAM for main memory

Set-rest flip-flops

A high voltage to S sets the top output to 1 and that cannot be undone lowering voltage at S - essentially storing the bit

A high voltage to R switches the output to 1 at the bottom and 0 at the top

D-type flip-flop

No set/reset - just a data input

Acts like a delay or 1 bit store

State changes only when clock is high

It is an RS latch with additonal NAND gates that make it simple to control. When Enable is 0, both control NAND gates will have a 1 output, and the RS latch will remain stable. When Enable = 1, Q will become equal to Data. If Data changes while Enable = 1, Q will also change. When Enable goes back to 0, the most recent value of D will remain on the Q output.

Shift register

Shifts output every clock cycle

Can be used to convert between serial and parallel

User visible registers

General purpose

• Usually 16 in modern CPUs

Data

Address only (less common)

• DO not store data

• Only a memory address

Condition codes in status register

• Sets of individual bits

• Can be read by programs but not usually set by them

• Code examples: carry, overflow, interrupt

Control registers

Usually hidden from user

E.g. MAR, MBR

Supervisor mode

Status controlled by status register

Allows privileged instructions to execute

Used by OS