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Most of its instructions assume that operands will be from the stack, and results placed in the stack. For a typical instruction like "Add," the computer takes both operands from the topmost most recent values of the stack.
The computer replaces those two values by the sum, calculated by the computer when it performs the "Add" instruction. Most stack instructions have only an opcode commanding an operation, with no additional fields to identify a constant, register or memory cell.
The stack easily holds more than two inputs or more than one result, so a richer set of operations can be computed. Integer constant operands are often pushed by separate Load Immediate instructions. Memory is often accessed by separate Load or Store instructions containing a memory address or calculating the address from values in the stack.
For speed, a stack machine often implements some part of its stack with registers. To execute quickly, operands of the arithmetic logic unit ALU may be the top two registers of the stack and the result from the ALU is stored in the top register of the stack.
Some stack machines have a stack of limited size, implemented as a register file. The ALU will access this with an index. This is slower, but the number of flip-flops is less, making a less-expensive, more compact CPU.
Its topmost N values may be cached for speed. A few machines have both an expression stack in memory and a separate register stack. In this case, software, or an interrupt may move data between them. The instruction set carries out most ALU actions with postfix reverse Polish notation operations that work only on the expression stack, not on data registers or main memory cells.
This can be very convenient for executing high-level languages, because most arithmetic expressions can be easily translated into postfix notation. In contrast, register machines hold temporary values in a small, fast array of registers.
Accumulator machines have only one general-purpose register. Belt machines use a FIFO queue to hold temporary values. Memory-to-memory machines do not have any temporary registers usable by a programmer. Stack machines may have their expression stack and their call-return stack separated or as one integrated structure.
If they are separated, the instructions of the stack machine can be pipelined with fewer interactions and less design complexity.
Usually it can run faster. Some technical handheld calculators use reverse Polish notation in their keyboard interface, instead of having parenthesis keys. This is a form of stack machine.
The Plus key relies on its two operands already being at the correct topmost positions of the user-visible stack. Advantages of stack machine instruction sets[ edit ] Very compact object code[ edit ] Stack machines have much smaller instructions than the other styles of machines.
Loads and stores to memory are separate and so stack code requires roughly twice as many instructions as the equivalent code for register machines.
The total code size in bytes is still less for stack machines[ citation needed ]. In stack machine code, the most frequent instructions consist of just an opcode selecting the operation. This can easily fit in 6 bits or less. The selection of operands from prior results is done implicitly by ordering the instructions.
In contrast, register machines require two or three register-number fields per ALU instruction to select operands; the densest register machines average about 16 bits per instruction. The instructions for accumulator or memory-to-memory machines are not padded out with multiple register fields.
Instead, they use compiler-managed anonymous variables for subexpression values. These temporary locations require extra memory reference instructions which take more code space than for the stack machine, or even compact register machines.
This can be by offsets from the current top-of-stack address, or by offsets from a stable frame-base register. Dense machine code was very valuable in the s, when main memory was very expensive and very limited even on mainframes. It became important again on the initially-tiny memories of minicomputers and then microprocessors.
Density remains important today, for smartphone applications, applications downloaded into browsers over slow Internet connections, and in ROMs for embedded applications. A more general advantage of increased density is improved effectiveness of caches and instruction prefetch.Computers have both advantages and disadvantages.
On one hand, they allow us to access the wealth of information that is available on the internet.
They also allow us . Laptop have both Advantages and Disadvantages. In this essay we’ll read all the advantages and disadvantages of using Laptop in attheheels.com before going towards benefits and drawbacks of Laptop we’ll read little definition. We are living in the world where technology almost surpassed humanity said by “Albert Einstein”.
It’s worthless to mention nowadays that technology is playing a very important role in every aspect of life. The questions below are from a variety of sources, and are recent IELTS essay questions from as reported either by recent test takers directly in the comments section on this page, or from an assortment of other sources across the web.
Apr 12, · List Of Latest PTE Essay Topics With Answers | PTE Essay Writing. Abstract “Growing concerns over climate change have highlighted the need to step up contribution of nuclear energy in the energy mix and to reduce the.