Memory & the fetch decode-execute cycle
- 1. Main Memory & Memory Addresses
- 2. Memory A memory chip is used to store all our data. A chip which is set next to the CPU as it makes direct contact with the CPU. • Volotile - stores data temporarily, It means that when the computer is switched off, that particular data is removed completely from the system. E.g. RAM • Non-volotile - stores data permanently, it means that when the computer is switched off all data remains stored in the system. E.g. ROM
- 3. Difference between ROM & RAM
- 4. What is Machine Code? Machine code is the only form of program instructions that the computer hardware can understand and execute directly. All other forms of computer language must be translated into machine code in order to be executed by the hardware. E.g. Assembly language Assembly language is a symbolic representation of machine code, which allows programmers to write programs in machine code without having to deal with the long binary strings.
- 5. Assembly Language is made up of op-codes and operands Instructions in assembly language are rather simple • An opcode is a single instruction that can be executed by the CPU. In machine language it is a binary or hexadecimal value such as 'b6' loaded into the instruction register e.g. Mov, add , jmp •Operands are manipulated by the opcode. MOV, AL, 34H The operands are the register named AL and the value 34 hex
- 7. THE FETCH – EXECUTE CYCLE Both the data and the program that acts upon that data are loaded into main memory (RAM) by the operating system. The CPU is now ready to do some work.
- 8. Steps of the Fetch/Execute Cycle: • Get the next instruction • Figure out what to do • Gathering the data needed to do it • Do it • Save the result, and • Repeat (billions of times/second)!
- 9. Fetch Cycle • The Program Counter (PC) contains the address of the next instruction to be fetched • The address contained in the PC is copied to the Memory Address Register (MAR). • The instruction is copied from the memory location contained in the MAR and placed in the Memory Buffer Register (MBR). • The entire instruction is copied from the MBR and placed in the Current Instruction Register (CIR) • The PC is incremented so that it points to the next instruction to be fetched
- 10. Execute Cycle • The address part of the instruction is placed in the MAR • The instruction executed is decoded and • The processor checks for interrupts (signals from devices or other sources seeking the attention of the processor) and either branches to the relevant interrupt service routine or starts the cycle again.
- 11. CU 100 101 100 PC Program Counter IR Multiply no. Instruction in 500, 501 Register ALU Acc Accumulator 378 Multiply no. in 500,501 101 Store result in 502 500 21 501 18 502 1.The PC contains the address of location 100 2.CU fetches instruction in location 100 3. Make a copy of the instruction into the IR 4. Increment the PC by 1 5. Activate the right circuits to execute the instruction
- 12. CU PC 100 102 101 Program Counter Store result Multiply no. Instruction IR inin 502 500, 501 Register ALU Acc Accumulator 378 Multiply no. in 500,501 101 Store result in 502 500 21 501 18 502 378 1. The PC contains the address of location 101 2. CU fetches instruction in location 101 3. A copy of the instruction is saved in the IR 4. Increment the PC 5. Activate the right circuits to execute the instruction