Briefly explain the differences between the physical registers and ISA registers

Question

Briefly explain the differences between the physical registers and ISA registers. Make sure to write one example per each. How many 8-bit adders are required to fully parallelized a 8-bit by 8-bit integer multiplication in hardware. Please draw the corresponding architecture. Calculate the CPI of the fully paralleled 16-bit integer multiplier, and the achievable compared to serial 16-bit integer multiplier. How many shift and subtract operations are required for dividing 10110101 to 1010. Make sure to write your approach for achieving the result.

Explanation / Answer

1.

Registers are normally at the top of memory hierarchy and provides the fastest way for the CPU to access data. Most of the Instruction Set Architectures (ISA) like ARM/x86/Power provides list of architectural registers which are encoded part of instructions and available for software. An underlying microarchitecture may decide to implement more physical registers for faster execution. (and map architectural registers using technique called register renaming). Registers are normally implemented using flip-flops and are hence costly.

Buffers are temporary storage and are more microarchitecture specific, normally not exposed to software or any user. Some common examples re-order buffer (that stores results of out of order execution of instructions untill committed) or fill buffers (that stores fill data from a cache) or any other interface storage to deal with flow control/credits etc.

Main memory : This is the physical memory that is connected directly to the CPU and can be accessed directly over a bus. It is also known as RAM. The access will be slower than registers and caches but faster than secondary memory.

The other parts of memory hierarchy of a CPU are caches (again multiple levels - L1/L2/L3) sitting between registers and main memory and secondary memory (sitting further down main memory)

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