Architecture 03 Flashcards
What is meant by word-addressed?
Where one address is assigned for each word
• Extra info is supplied to refer to a specific byte within a word
What is meant by byte-addresses?
Where one address is assigned for each 8 bit byte
• Lowest address in the word stands for the word (i.e from this address you can infer the rest of the addresses in the word)
• This is now the standard
What is meant by little-endian?
A characterisation of a system which stores and transmits bytes of an integer from least significant to most significant bit
What is meant by big-endian?
A characterisation of a system which stores and transmits bytes of an integer from most significant to least significant bit (like humans)
What is meant by the addressing mode?
How an ISA allows instructions to access memory
What is meant by immediate mode addressing?
When a value is specified in an instruction itself
• Used for constants
What is meant by direct mode addressing?
When a value is at a fixed address specified in an instruction
• Used for global variables
What is meant by indirect mode addressing?
When a value is at a fixed address that is in a register
• Extra power, but a = b[i] is cumbersome to implement
What is meant by index mode addressing?
When a value is at a fixed address plus index obtained from register
• Allows implementation of a = b[i] in one instruction
What are the advantages of orthogonality?
- Simplifies code development
* More powerful instructions and fewer instructions are required to do the same thing
What are the disadvantages of orthogonality?
- Chips may need to be bigger and more complex
- Instructions may be slower if multiple memory access is done in them
- Instructions may be longer as more operands may be required
What is CISC?
A Complex Instruction Set Computer
• No orthogonality
• Lots of addressing modes
• Variable-length instructions
What is RISC?
A Reduced Instruction Set Computer
• Fixed-length instructions (usually 3 operands)
• Many registers
• Often register-register and few addressing modes
What are the advantages of RISC over CISC?
- More instructions done per second
* Design is more compact and has more compatability
What are the advantages of CISC over RISC?
- Less instructions needed for a specific task
* More elegant and simple instructions
