What are the two majors factors to be considered while designing Instruction Set Architecture ?
I know what ISA is . But What are the factors to be considered? I already reviewed Wikipedia but it doesn't help much.
I found this as design issues for ISA.
- Backward Compatibility
- Are interrupts needed?
But are this two factors I am bit confused ! Please help any one ! Preparing For exams of Computer Organization and Architecture
you can read full article from here The Importance of the Design of the Instruction Set
In this chapter we will be exploring one of the most interesting and important aspects of CPU design: the design of the CPU's instruction set. The instruction set architecture (or ISA) is one of the most important design issues that a CPU designer must get right from the start. Features like caches, pipelining, superscalar implementation, etc., can all be grafted on to a CPU design long after the original design is obsolete. However, it is very difficult to change the instructions a CPU executes once the CPU is in production and people are writing software that uses those instructions. Therefore, one must carefully choose the instructions for a CPU.
You might be tempted to take the "kitchen sink" approach to instruction set design1 and include as many instructions as you can dream up in your instruction set. This approach fails for several reasons we'll discuss in the following paragraphs. Instruction set design is the epitome of compromise management. Good CPU design is the process of selecting what to throw out rather than what to leave in. It's easy enough to say "let's include everything." The hard part is deciding what to leave out once you realize you can't put everything on the chip.
Nasty reality #1: Silicon real estate. The first problem with "putting it all on the chip" is that each feature requires some number of transistors on the CPU's silicon die. CPU designers work with a "silicon budget" and are given a finite number of transistors to work with. This means that there aren't enough transistors to support "putting all the features" on a CPU. The original 8086 processor, for example, had a transistor budget of less than 30,000 transistors. The Pentium III processor had a budget of over eight million transistors. These two budgets reflect the differences in semiconductor technology in 1978 vs. 1998.
Nasty reality #2: Cost. Although it is possible to use millions of transistors on a CPU today, the more transistors you use the more expensive the CPU. Pentium IV processors, for example, cost hundreds of dollars (circa 2002). A CPU with only 30,000 transistors (also circa 2002) would cost only a few dollars. For low-cost systems it may be more important to shave some features and use fewer transistors, thus lowering the CPU's cost.
Nasty reality #3: Expandability. One problem with the "kitchen sink" approach is that it's very difficult to anticipate all the features people will want. For example, Intel's MMX and SIMD instruction enhancements were added to make multimedia programming more practical on the Pentium processor. Back in 1978 very few people could have possibly anticipated the need for these instructions.
Nasty reality #4: Legacy Support. This is almost the opposite of expandability. Often it is the case that an instruction the CPU designer feels is important turns out to be less useful than anticipated. For example, the LOOP instruction on the 80x86 CPU sees very little use in modern high-performance programs. The 80x86 ENTER instruction is another good example. When designing a CPU using the "kitchen sink" approach, it is often common to discover that programs almost never use some of the available instructions. Unfortunately, you cannot easily remove instructions in later versions of a processor because this will break some existing programs that use those instructions. Generally, once you add an instruction you have to support it forever in the instruction set. Unless very few programs use the instruction (and you're willing to let them break) or you can automatically simulate the instruction in software, removing instructions is a very difficult thing to do.
Nasty reality #4: Complexity. The popularity of a new processor is easily measured by how much software people write for that processor. Most CPU designs die a quick death because no one writes software specific to that CPU. Therefore, a CPU designer must consider the assembly programmers and compiler writers who will be using the chip upon introduction. While a "kitchen sink" approach might seem to appeal to such programmers, the truth is no one wants to learn an overly complex system. If your CPU does everything under the sun, this might appeal to someone who is already familiar with the CPU. However, pity the poor soul who doesn't know the chip and has to learn it all at once.
These problems with the "kitchen sink" approach all have a common solution: design a simple instruction set to begin with and leave room for later expansion. This is one of the main reasons the 80x86 has proven to be so popular and long-lived. Intel started with a relatively simple CPU and figured out how to extend the instruction set over the years to accommodate new features.