1.2: Micro-architectures - Achievements Connections by Wires and Microprogrammed

Introduction

This section introduces the learners to the micro-architecture of the computer. That is the resources and methods used to achieve specifications.

Activity Details

Micro-architecture is the term used to describe the resources and methods used to achieve architecture specification. The term typically includes the way in which these resources are organized as well as the design techniques used in the processor to reach the target cost and performance goals. The micro-architecture essentially forms a specification for the logical implementation.

Also called computer organization and sometimes abbreviated as μarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be implemented with different micro-architectures; implementations may vary due to different goals of a given design or due to shifts in technology.

The micro-architecture is related to, but not the same as, the instruction set architecture. Micro- architectural elements may be everything from single logic gates, to registers, lookup tables, multiplexers, counters, etc., to complete ALUs, floating point units (FPU) and even larger elements.

A few important points:

• A single micro-architecture can be used to implement many different instruction sets, by means of changing the control store.
• Two machines may have the same micro-architecture, and so the same block diagram, but completely different hardware implementations. This manages both the level of the electronic circuitry and even more the physical level of manufacturing (of both ICs and/or discrete components).
• Machines with different micro-architectures may have the same instruction set architecture, and so both are capable of executing the same programs. New micro-architectures and/or circuitry solutions, along with advances in semiconductor manufacturing, are what allow newer generations of processors to achieve higher performance.

The pipelined datapath is the most commonly used datapath design in micro-architecture today. This technique is used in most modern microprocessors, microcontrollers, and DSPs. The pipelined architecture allows multiple instructions to overlap in execution, much like an assembly line.

The pipeline includes several different stages which are fundamental in micro-architecture designs. Some of these stages include instruction fetch, instruction decode, execute, and write back. Some architectures include other stages such as memory access. The design of pipelines is one of the central micro-architectural tasks.

Execution units are also essential to micro-architecture. Execution units include arithmetic logic units (ALU), floating point units (FPU), load/store units, branch prediction, and SIMD.

These units perform the operations or calculations of the processor. The choice of the number of execution units, their latency and throughput is a central micro-architectural design task.

The size, latency, throughput and connectivity of memories within the system are also micro- architectural decisions.

System-level design decisions such as whether or not to include peripherals, such as memory controllers, can be considered part of the micro-architectural design process. This includes decisions on the performance-level and connectivity of these peripherals.

Unlike architectural design, where achieving a specific performance level is the main goal, micro-architectural design pays closer attention to other constraints. Since micro-architecture design decisions directly affect what goes into a system, attention must be paid to such issues as:

• Chip area/cost.
• Power consumption.
• Logic complexity.
• Ease of connectivity.
• Manufacturability.
• Ease of debugging.
• Testability.

Conclusion

This section has highlighted the micro-architecture. It discussed the resources and methods used to achieve the architecture.

Assessment

1. Outline with an explanation the micro-architecture

Micro-architecture is used to describe the units that were controlled by the micro-program words. Micro-architecture is related to, but not the same as, the instruction set architecture.

The instruction set architecture is near to the programming model of a processor as seen by an assembly language programmer or compiler writer, which includes the execution model, processor registers, address and data formats etc. The micro-architecture (or computer organization) is mainly a lower level structure and therefore manage a large number of details that are hidden in the programming model. It describes the inside parts of the processor and how they work together in order to implement the architectural specification.

Micro-architectural elements may be everything from single logic gates, to registers, lookup tables, multiplexers, counters, etc., to complete ALUs, FPUs and even larger elements. The electronic circuitry level can, in turn, be subdivided into transistor-level details, such as which basic gate-building structures are used and what logic implementation types (static/dynamic, number of phases, etc.) are chosen, in addition to the actual logic design used built them.