Instruction decoder logicworks 5 code#
Instruction decoder logicworks 5 Pc#

Instruction decoder logicworks 5 Pc#

It is important to note that the PC and the MAR (address buffer) have a fixed length of so many bits. Program counter identifies the address in memory to be accessed. The instructions in a program are stored in sequential memory locations.įigure 6.7. After the instruction is executed, the PC is incremented (add 1) to the next address in sequence, or 44. The address in the PC is applied to the memory, causing the instruction in location 43 to be fetched and executed. If a program begins with an instruction stored in memory location 43, the PC is first loaded with the address 43. It stores a binary word that is used as the address for accessing the instructions in a program. The PC is really a counter and a register. The CPU decodes the instruction, and then executes it.

Instruction decoder logicworks 5 code#

The instruction word is also called the op code or operation code. The instruction is a binary word or code that defines a specific operation to be performed. When the CPU fetches an instruction from memory, it is temporarily stored in the IR. The IR is used to store the instruction word. Frenzel Jr., in Electronics Explained (Second Edition), 2018 Control UnitĬPUs also have several other registers, including the instruction register (IR) the program counter (PC), also called the instruction counter and the memory address register (MAR), also called the address buffer. For example, to move an 8-bit data word from file register 0C to W requires the syntax MOVF 0C,W. This bit must be 0 to direct the data into W, the usual operation. In the MOVF instruction the operation code is 001000, and the file register address is needed as before to identify the data source register. Only 7 bits are used for the register address, allowing a maximum of 2 7 = 128 registers to be addressed. In the MOVWF instruction, the operation code is 0000001 (7 bits) and ‘f’ bits specify the file register address. In the MOVLW instruction, the operation code is the high 4 bits (1100), ‘x’ are ‘don't care’ bits, and ‘k’ represents the literal bits, the low byte of the instruction.

If we look at the ‘move’ instruction codes quoted in the instruction set, we can see the difference in the code structure for the three move instructions:

The operand will be the address of the file register (00 to 4F) required. If the instruction is MOVWF, the control lines will be set up to copy the contents of W to the specified file register via the internal data bus. If, for example, the instruction is MOVLW (Move a Literal into W), the control lines will be set up to feed the literal operand to W via literal data bus to the multiplexer and ALU. The operand provides a literal, file register address or program address, which will be used by the instruction. The instruction decoder logic converts the op-code bits into settings for all the internal control lines. The 14-bit instructions stored in program memory are copied to the instruction register for decoding each instruction contains both the operation code and operand. The program execution section of the MCU contains the instruction register, instruction decoder, and timing and control logic. Martin Bates, in PIC Microcontrollers (Third Edition), 2011 5.2.2 Instruction Execution