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| en:multiasm:cs:chapter_3_4 [2025/12/02 13:05] – [Execution unit] ktokarz | en:multiasm:cs:chapter_3_4 [2026/01/10 20:11] (current) – pczekalski |
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| ===== Control unit ===== | ===== Control unit ===== |
| The function of the control unit, also known as the instruction processor, is to fetch, decode and execute instructions. It also generates signals to the execution unit if the instruction being executed requires so. It is a synchronous and sequential unit. Synchronous means that it changes state synchronously with the clock signal. Sequential means that the next state depends on the states at the inputs and the current internal state. As inputs, we can consider not only physical signals from other units of the computer but also the code of the instruction. To ensure that the computer behaves the same every time it is powered on, the execution unit is set to the known state at the beginning of operation by the RESET signal. | The function of the control unit, also known as the instruction processor, is to fetch, decode and execute instructions. It also generates signals to the execution unit if the instruction being executed requires it. It is a synchronous and sequential unit. Synchronous means that it changes state synchronously with the clock signal. Sequential means that the next state depends on the states at the inputs and the current internal state. As inputs, we can consider not only physical signals from other units of the computer but also the instruction code. To ensure that the computer behaves the same every time it is powered on, the execution unit is set to the known state at the beginning of operation by the RESET signal. |
| A typical control unit contains some essential elements: | A typical control unit contains some essential elements: |
| * Instruction register (IR). | * Instruction register (IR). |
| - If the instruction requires the execution unit operation, the control unit generates signals to control it. In cooperation with the execution unit, it can also read data from or write data to the memory. | - If the instruction requires the execution unit operation, the control unit generates signals to control it. In cooperation with the execution unit, it can also read data from or write data to the memory. |
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| The control unit works according to the clock signal generator cycles known as main clock cycles. With every clock cycle, some internal operations are performed. One such operation is reading or writing the memory, which sometimes requires more than a single clock cycle. Single memory access is known as a machine cycle. As instruction execution sometimes requires more than one memory access and other actions, the execution of the whole instruction is named an instruction cycle. Summarising, one instruction execution requires one instruction cycle and several machine cycles, each composed of a few main clock cycles. Modern advanced processors are designed in such a way that they are able to execute a single instruction (sometimes even more than one) every single clock cycle. This requires a more complex design of a control unit, many execution units and other advanced techniques, which makes it possible to process more than one instruction at a time. | The control unit operates according to the clock signal generator's cycles, known as main clock cycles. With every clock cycle, some internal operations are performed. One such operation is reading or writing the memory, which sometimes requires more than a single clock cycle. Single memory access is known as a machine cycle. As instruction execution sometimes requires more than one memory access and other actions, the execution of the whole instruction is named an instruction cycle. Summarising, one instruction execution requires one instruction cycle and several machine cycles, each composed of a few main clock cycles. Modern advanced processors are designed in such a way that they are able to execute a single instruction (sometimes even more than one) every single clock cycle. This requires a more complex control unit design, many execution units, and other advanced techniques, which enable processing more than one instruction at a time. |
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| The control unit also accepts input signals from peripherals, enabling interrupts and direct memory access mechanisms. For proper return from the interrupt subroutine, the control unit uses a special register called the stack pointer. Interrupts and direct memory access mechanisms will be explained in detail in further chapters. | The control unit also accepts input signals from peripherals, enabling interrupts and direct memory access mechanisms. For proper return from the interrupt subroutine, the control unit uses a special register called the stack pointer. Interrupts and direct memory access mechanisms will be explained in detail in further chapters. |
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| The arithmetic logic unit (ALU) is the element that performs logical and arithmetical calculations. It uses data coming from registers, the accumulator or from memory. Data coming from memory for arithmetic and logic instructions is stored in the temporal register. The result of calculations is stored back in the accumulator, another register or memory. In some legacy CISC processors, the only possible place for storing the result is the accumulator. | The arithmetic logic unit (ALU) is the element that performs logical and arithmetical calculations. It uses data coming from registers, the accumulator or from memory. Data coming from memory for arithmetic and logic instructions is stored in the temporal register. The result of calculations is stored back in the accumulator, another register or memory. In some legacy CISC processors, the only possible place for storing the result is the accumulator. |
| Besides the result, ALU also returns some additional information about the calculations. It modifies the bits in the flag register, which comprises flags that are modified according to the results from arithmetic and logical operations. For example, if the result of the addition operation is too large to be stored in the resulting argument, the carry flag is set to indicate such a situation. | Besides the result, ALU also returns some additional information about the calculations. It modifies the bits in the flag register, which contains flags that are updated based on the results of arithmetic and logical operations. For example, if the result of the addition operation is too large to be stored in the resulting argument, the carry flag is set to indicate such a situation. |
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| Typically, the flags register includes: | Typically, the flags register includes: |
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| The flags are used as conditions for decision-making instructions (like //if// statements in some high-level languages). | The flags are used as conditions for decision-making instructions (like //if// statements in some high-level languages). |
| The flags register can also implement some control flags to enable/disable processor functionalities. An example of such a flag can be the Interrupt Enable flag from the 8086 microprocessor. | The flags register can also implement some control flags to enable/disable processor functionalities. An example of such a flag is the Interrupt Enable flag on the 8086 microprocessor. |
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| ===== Registers ===== | ===== Registers ===== |
