What four categories comprise user registers?
General purpose
Data
Address
Condition code
Describe a user register
Allow machine code to access main memory
What do general registers do?
As the name suggests. Particularly flexible for operands.
What do data registers do?
Holding data
What do address registers do?
Can be somewhat general or for specific addressing modes.
Describe control and status registers
Used by control unit.
Determine how a program executes.
What two things could you consider when designing register architecture?
Amount of registers
Size of registers
What should you be mindful of when considering the amount of registers?
That more is not necessarily better.
If you have fewer, you may have to do more memory access requests.
What should you be mindful of when consider register size.
- Registers that deal with addresses should be at least large enough to hold the largest address.
- Data registers should be able to hold data of multiple types.
What do condition codes?
Bits set by the CPU related to a particular instruction.
What are the four essential registers for instruction
Program Counter
Memory Buffer Register
Memory Address Register
Instruction Register
What does the Program Counter hold?
Next instruction to be taken.
When does the Program Counter update?
When the currently executing instruction finishes.
When a conditional branch code is taken.
What does the Memory Buffer Register do?
Holds data that has to be worked on or sent through the data bus.
What does the Memory Address Register do?
Specifies the address of what to fetch or store in memory.
Where part of the bus does the Memory Buffer Register connect to?
Data bus
What part of the bus does the Memory Address Register connect to?
Address bus
What does the Instruction Register do?
It contains the most recently fetched instruction (the thing that is running).
What is the simple order of register use
1) Fetched instruction goes into instruction register
2) Opcode and operand are specified
3) Data exchanged with MAR and MBR
4) Program counter increments
What is the Program Status Word?
It’s a register that contains status information.
What things could be included in the Program Status Word?
Condition codes
Sign of a number
Whether the CPU is in supervisor or user mode
