machine code/fetch execute cycle Flashcards
What voltages are typically used for binary values?
1 = 5 volts 0 = 0 volts
What is a machine code instruction?
A binary code that a machine can interpret and execute
What is assembly language?
A mnemonic form of machine code (easier for people to read but still a one to one relationship)
What are compiled high-level language programs?
Programs translated into machine code before they can be executed on a digital computer
What is an executable file (.exe) written in?
machine code
What parts does a machine code instruction consist of?
the op-code (operation code) and operand
What is the op-code?
The part of a machine code instruction that includes the basic machine operation
(eg ADD)
What is the operand?
The part of a machine code instruction that represents one item of data or its address to be operated upon
What does the instruction LOAD #3 mean?
place the value 3 into accumulator
What does the instruction STORE 13 mean?
put what is in the accumulator into the memory loacation with address 13
What does the instruction ADD 13 mean?
Add up the contents of the accumulator and the contents of memory location 13 (the result goes in the accumulator)
Which register do ADD, LOAD and STORE use?
The accumulator
How are data and addresses represented in assembly code?
In decimal or hexadecimal
What is a memory dump?
A copy of the main memory contents - viewed by programmers to see where an error has occurred
What is the instruction set?
The set of bit patterns or binary codes for the machine operations that a processor has been designed to perform
i.e. every possible operation that can be carried out by a particular processor
If there are 4 bits allocated to the op-code then how large will the instruction set be?
2^4 = 16 so there are 16 possible operations in the instruction set
How can an op-code be divided up?
Into the basic instruction and which addressing mode will be used
What is an addressing mode?
Specifies in the op-code part, shows how the operand part should be interpreted
What are the three addressing modes?
-register transfer involving main memory =main memory address will be specified -operand to register transfer =A value not address will be specified -operand part not used
Give an example of an instruction where the operand part is not used
HALT
just stops the program executing, does not need any data or addresses
What is the name for fetching, decoding and executing an instruction repeatedly in a stored program computer?
fetch-execute cycle
Which registers are always involved in the fetch execute cycle?
- program counter
- memory address register
- memory buffer register
- current instruction register
What is an alternative name for the memory buffer register?
memory data register
What are the four steps in the fetch phase of the fetch execute cycle?
- The address of the next instruction to be executed goes from the program counter to the memory address register
- The instruction held in the address is given to the memory buffer register
- The program counter is incremented by one
- The contents of the memory buffer register is copied to the current instruction register
What are the two steps in the execute phase of the fetch execute cycle?
- Instruction i the current instruction register is decoded
- Instruction is executed
What is register transfer notation?
used to show how things are passed between different registers in a processor
How would the contents of program counter be written in register transfer notation?
[PC]
contents of = square brackets
How is assigned to represented in register transfer notation?
<—-
the same way as in pseudocode
What does x;y mean in register transfer notation?
instruction x and instruction y should be executed at the same time
What does SP stand for?
stack pointer register
What does PC stand for?
program counter register
What does SR stand for?
status register
What does ACC stand for?
accumulator
What does CIR stand for?
current instruction register
What does MAR stand for?
memory address register
What does MBR stand for?
memory buffer register
Give a basic description of the steps taken in the fetch execute cycle
- Memory address register takes the value of the program counter (ie. the address of the next instruction to be executed)
- control circuit sends down the control bus whether read or write from that address is needed
- Data from the memory address goes up the data bus to the memory buffer register
- Instructions go to the current instruction register
- program counter is incremented
- Instructions are decoded and executed
- Data goes to accumulator/general purpose registers (the top free register)
