What is VE (Vertical Encoding)

VE (Vertical Encoding): Simplifying Control in Microprogrammed Processors

Vertical Encoding (VE) is a technique used within the world of microprogrammed control units (CU) for processors. It focuses on encoding control signals for various processing operations using a single instruction word. This approach offers a simpler and more compact way to define control logic compared to its counterpart, horizontal microprogramming.

Understanding Microprogrammed Control Units (CU):

The CU acts as the brain of a processor, directing the execution of instructions by generating control signals for various functional units like the Arithmetic Logic Unit (ALU) and registers. Traditionally, CUs were implemented using hardwired logic circuits. However, microprogramming introduced a more flexible approach.

Microprogrammed Control vs. Hardwired Logic:

• Hardwired Logic: Control signals are directly generated by fixed logic circuits within the CU. This approach offers high performance but lacks flexibility. Modifying control logic requires physical changes to the circuitry, making it a tedious and time-consuming process.
• Microprogrammed Control: The CU utilizes a control store (typically a read-only memory - ROM) that holds microinstructions. These microinstructions define the control signals for each operation. This approach offers greater flexibility as control logic can be modified by updating the microcode stored in the ROM.

Horizontal vs. Vertical Microprogramming:

There are two main approaches to defining microinstructions in a microprogrammed CU:

• Horizontal Microprogramming: Each microinstruction explicitly defines the control signal for every functional unit within the processor. This results in lengthy microinstructions and requires a larger control store.
• Vertical Microprogramming (VE): Uses a single instruction word to represent an entire micro-operation. VE employs a control decoder that interprets the encoded information within the instruction word and generates the necessary control signals for various functional units.

Benefits of Vertical Encoding:

• Reduced Control Store Size: By encoding information, VE allows for more compact microinstructions, leading to a smaller control store. This translates to cost savings and potentially faster access times.
• Simpler Microcode Development: VE simplifies microcode development as programmers only need to focus on the operation itself, and the control decoder handles generating the appropriate control signals.

Drawbacks of Vertical Encoding:

• Limited Flexibility: VE offers less flexibility compared to horizontal microprogramming. Adding new control signals requires modifying the control decoder logic, which can be more complex than simply adding bits to a horizontal microinstruction.
• Decoder Complexity: The control decoder in VE adds another layer of complexity to the CU design. This can potentially impact performance and require additional design effort.

Applications of Vertical Encoding:

VE is primarily used in processors where simplicity and cost are key considerations. It is often found in embedded systems and microcontrollers where a balance between performance and resource limitations is crucial.

The Future of VE:

While horizontal microprogramming might offer more flexibility, VE remains a relevant technique for simpler processors due to its advantages in code density and potentially lower cost. However, as processor architectures evolve and require more complex control functionalities, the use of VE might decline in favor of more flexible approaches.

Understanding VE equips you with valuable knowledge about a fundamental concept in microprogrammed control units. It highlights the trade-offs between simplicity, flexibility, and performance in processor design.