REGISTERS

The basic purpose of a computer is to perform operations, and operations need operands. Operands are the data on which we want to perform a certain operation. Consider the addition operation; it involves adding two numbers to get their sum. We can have precisely one address on the address bus and consequently precisely one element on the data bus. At the very same instant the second operand cannot be brought inside the processor. As soon as the second is selected, the first operand is no longer there. For this reason there are temporary storage places inside the processor called registers. Now one ope rand can be read in a register and added into the other which is read directly from the memory. Both are made accessible at one instance of time, one from inside the processor and one from outside on the data bus. The result can be written to at a distinct location as the operation has completed and we can access a different memory cell. Sometimes we hold both operands in registers for the sake of efficiency as what we can do inside the processor is undoubtedly faster than if we have to go outside and bring the second operand.

Registers are like a scratch pad ram inside the processor and their operation is very much like normal memory cells. They have precise locations and remember what is placed inside them. They are used when we need more than one data element inside the processor at one time. The concept of registers will be further elaborated as we progress into writing our first program.

Memory is a limited resource but the number of memory cells is large. Registers are relatively very small in number, and are therefore a very scarce and precious resource. Registers are more than one in number, so we have to precisely identify or name them. Some manufacturers number their registers like r0, r1, r2, others name them like A, B, C, D. Naming is useful since the registers are few in number. This is called the nomenclature of the particular architecture. Still other manufacturers name their registers according to their function like X stands for an index register. This also informs us that there are special functions of registers as well, some of which are closely associated to the particular architecture. For example index registers do not hold data instead they are used to hold the address of data. There are other functions as well and the whole spectrum of register functionalities is quite large. However most of the details will become clear as the registers of the Intel architecture are discussed in detail.

Accumulator

There is a central register in every processor called the accumulator. Traditionally all mathematical and logical operations are performed on the accumulator. The word size of a processor is defined by the width of its accumulator. A 32bit processor has an accumulator of 32 bits.

Pointer, Index, or Base Register

The name varies from manufacturer to manufacturer, but the basic distinguishing property is that it does not hold data but holds the address of data. The rationale can be understood by examining a “for” loop in a higher level language, zeroing elements in an array of ten elements locate d in consecutive memory cells. The location to be zeroed changes with every iteration. That is the address where the operation is performed is changing. Index register is used in such a situation to hold the address of the current array location. Now the value in the index register cannot be treated as data, but it is the address of data. In general whenever we need access to a memory location whose address is not known until runtime we need an index register. Without this register we would have needed to explicitly code each iteration separately.

In newer architectures the distinction between accumulator and index registers has become vague. They have general registers which are more versatile and can do both functions. They do have some specialized behaviors but basic operations can be done on all general registers.

Flags Register or Program Status Word

This is a special register in every architecture called the flags register or the program status word. Like the accumulator it is an 8, 16, or 32 bits regi ster but unlike the accumulator it is meaningless as a unit, rather the individual bits carry different meanings. The bits of the accumulator work in parallel as a unit and each bit mean the same thing. The bits of the flags register work independently and individually, and combined its value is meaningless.

An example of a bit commonly present in the flags register is the carry flag. The carry can be contained in a single bit as in binary arithmetic the carry can only be zero or one. If a 16bit number is added to a 16bit accumulator, and the result is of 17 bits the 17th bit is placed in the carry bit of the flags register. Without this 17th bit the answer is incorrect. More examples of flags will be discussed when dealing with the Intel specific register set.

Program Counter or Instruction Pointer

Everything must translate into a binary number for our dumb processor to understand it, be it an operand or an operation itself. Therefore the instructions themselves must be translated into numbers. For example to add numbers we understand the word “add.” We translate this word into a number to make the processor understand it. This number is the actual instruction for the computer. All the objects, inheritance and encapsulation constructs in higher level languages translate down to just a number in assembly language in the end. Addition, multiplication, shifting; all big programs are made using these simple building blocks. A number is at the bottom line since this is the only thing a computer can understand.

A program is defined to be “an ordered set of instructions.” Order in this definition is a key part. Instructions run one after another, first, second, third and so on. Instructions have a positional relationship. The whole logic depends on this positioning. If the computer executes the fifth instructions after the first and not the second, all our logic is gone. The processor should ensure this ordering of instructions. A special register exists in every processor called the program counter or the instruction pointer that ensures this ordering. “The program counter holds the address of the next instruction to be executed.” A number is placed in the memory cell pointed to by this register and that number tells the processor which instruction to execute; for example 0xEA, 255, or 152. For the processor 152 might be the add instruction. Just this one number tells it that it has to add, where its operands are, and where to store the result. This number is called the opcode. The instruction pointer moves from one opcode to the next. This is how our program executes and progresses. One instruction is picked, its operands are read and the instruction is executed, then the next instruction is picked from the new address in instruction pointer and so on.

Remembering 152 for the add operation or 153 for the subtract operation is difficult. To make a simple way to remember difficult things we associate a symbol to every number. As when we write “add” everyone understands what we mean by it. Then we need a small program to convert this “add” of ours to 152 for the processor. Just a simple search and replace operation to translate all such symbols to their corresponding opcodes. We have mapped the numeric world of the processor to our symbolic world. “Add” conveys a meaning to us but the number 152 does not. We can say that add is closer to the programmer’s thinking. This is the basic motive of adding more and more translation layers up to higher level languages like C++ and Java and Visual Basic. These symbols are called instruction mnemonics. Therefore the mnemonic “add a to b” conveys more information to the reader. The dumb translator that will convert these mnemonics back to the original opcodes is a key program to be used throughout this course and is called the assembler.