Microprocessor-Assembly Level Language Programming: Introduction Assembly Level Language.

The oldest non-machine language, allowing for a more human readable method of writing programs than writing in binary bit patterns

An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture. A utility program called an assembler is used to translate assembly language statements into the target computer's machine code. The assembler performs a more or less isomorphic translation (a one-to-one mapping) from mnemonic statements into machine instructions and data. This is in contrast with high-level languages, in which a single statement generally results in many machine instructions. Typically a modern assembler creates object code by translating assembly instruction mnemonics into opcodes, and by resolving symbolic names for memory locations and other entities.

Number of passes

There are two types of assemblers based on how many passes through the source are needed to produce the executable program.

One-pass assemblers go through the source code once and assume that all symbols will be defined before any instruction that references them.
Two-pass assemblers create a table with all symbols and their values in the first pass, then use the table in a second pass to generate code. The assembler must at least be able to determine the length of each instruction on the first pass so that the addresses of symbols can be calculated.

The first-generation programming language, or 1GL, is machine code. It is the only language a microprocessor can process directly without a previous transformation. Currently, programmers almost never write programs directly in machine code, because it requires attention to numerous details which a high-level language would handle automatically, and also requires memorizing or looking up numerical codes for every instruction that is used. For this reason, second generation programming languages provide one abstraction level on top of the machine code.

The second-generation programming language, or 2GL, is assembly language. It is considered a second-generation language because while it is not a microprocessor's native language, an assembly language programmer must still understand the microprocessor's unique architecture (such as its registers andinstructions).

kinds of processors

Processors can broadly be divided into the categories of: CISC, RISC, hybrid, and special purpose.

Complex Instruction Set Computers (CISC) have a large instruction set, with hardware support for a wide variety of operations. In scientific, engineering, and mathematical operations with hand coded assembly language (and some business applications with hand coded assembly language), CISC processors usually perform the most work in the shortest time.

Reduced Instruction Set Computers (RISC) have a small, compact instruction set. In most business applications and in programs created by compilers from high level language source, RISC processors usually perform the most work in the shortest time.

Hybrid processors are some combination of CISC and RISC approaches, attempting to balance the advantages of each approach.

Special purpose processors are optimized to perform specific functions. Digital signal processors, graphics chips, and various kinds of co-processors are the most common kinds of special purpose processors.

executable instructions

There are four general classes of machine instructions. Some instructions may have characteristics of more than one major group. The four general classes of machine instructions are: computation, data transfer, sequencing, and environment control.

“Computation: Implements a function from n-tuples of values to m-tuples of values. The function may affect the state. Example: A divide instruction whose arguments are a single-length integer divisor and a double-length integer dividend, whose results are a single-length integer quotient and a single-length integer remainder, and which may produce a divide check interrupt.” —Compiler Construction, by William M. Waite and Gerhard Goos, page 52^b5
“Data Transfer: Copies information, either within one storage class or from one storage class to another. Examples: A move instruction that copies the contents of one register to another; a read instruction that copies information from a disc to main storage.” —Compiler Construction, by William M. Waite and Gerhard Goos, page 52^b5
“Sequencing: Alters the normal execution sequence, either conditionally or unconditionally. Examples: a halt instruction that causes execution to terminate; a conditional jump instruction that causes the next instruction to be taken from a given address if a given register contains zero.” —Compiler Construction, by William M. Waite and Gerhard Goos, page 53^b5
“Environment control: Alters the environment in which execution is carried out. The lateration may involve a trasnfer of control. Examples: An interrupt disable instruction that prohibits certain interrupts from occurring; a procedure call instruction that updates addressing registers, thus changing the program’s addressing environment.”

Data movement instructions move data from one location to another. The source and destination locations are determined by the addressing modes, and can be registers or memory.

The basic four integer arithmetic operations are addition, subtraction, multiplication, and division. Arithmetic operations can be signed or unsigned(unsigned is useful for effective address computations). Some older processors don’t include hardware multiplication and division. Some processors don’t include actual multiplication or division hardware, instead looking up the answer in a massive table of results embedded in the processor.

For most processors, integer arithmetic is faster than floating point arithmetic. This can be reversed in special cases such digital signal processors.

Binary coded decimal (BCD) is a method for implementing lossless decimal arithmetic (including decimal fractions) on a binary computer. The most obvious uses involve money amounts where round-off error from using binary approximations is unacceptable. Some early computers used BCD exclusively.

Decimal digits (0-9) can be encoded in a nibble (half a byte), with some left over bit patterns (hexadecimal A-F). In BCD operations, the processor performs ordinary binary computations, then adjusts the result to conform to BCD. For example, if you add the binary number 5 (bit pattern 0101) to binary number 6 (bit pattern 0110), you get the binary result of 11 (bit pattern 1011, or hexadecimal B). With BCD arithmetic, the processor would adjust the result to make it into a valid BCD result (which in this case would be bit pattern 0001 0001).

Data conversion instructions change data from one format to another.

A sign extension operation takes a small value and sign extends it to a larger storage format

Logical instructions typically work on a bit by bit basis, although some processors use the entire contents of the operands as whole flags (zero or not zero input, zero or negative one output). Typical logical operations include logical negation or logical complement (NOT), logical and (AND), logical inclusive or (OR or IOR), and logical exclusive or (XOR or EOR). Logical tests are a comparison of a value to a bit string (or operand treated as a bit string) of all zeros. Some processors have an instruction that sets or clears a bit or byte in registers or memory based on the processor condition codes.

Shift and rotate instructions move bit strings (or operand treated as a bit string).

Shift instructions move a bit string (or operand treated as a bit string) to the right or left, with excess bits discarded (although one or more bits might be preserved in flags). In arithmetic shift left or logical shift left zeros are shifted into the low-order bit. In arithmetic shift right the sign bit (most significant bit) is shifted into the high-order bit. In logical shift right zeros are shifted into the high-order bit.

Rotate instructions are similar to shift instructions, ecept that rotate instructions are circular, with the bits shifted out one end returning on the other end. Rotates can be to the left or right. Rotates can also employ an extend bit for multi-precision rotates.

A swap instruction swaps the high and low order portions of a register or contents of a series of memory locations.

Bit manipulation instructions manipulate a specific bit of a bit string (or operand treated as a bit string). Bit clear changes the specified bit to zero. Bit setchanges the specified bit to one. Bit change modifies a specified bit, clearing a one bit to zero and setting a zero bit to one. In some processors, the value of the bit before modification is tested. Bit test examines the value of a specified bit.

assembly/high level language interface

Because so many search requests are asking about how to interface between high level languages and assembly language, I have added this brief discussion.

program control

Program control instructions change or modify the flow of a program.

The most basic kind of program control is the unconditional branch or unconditional jump. Branch is usually an indication of a short change relative to the current program counter. Jump is usually an indication of a change in program counter that is not directly related to the current program counter (such as a jump to an absolute memory location or a jump using a dynamic or static table), and is often free of distance limits from the current program counter.

The pentultimate kind of program control is the conditional branch or conditional jump. This gives computers their ability to make decisions and implement both loops and algorithms beyond simple formulas.

Most computers have some kind of instructions for subroutine call and return from subroutines.

There are often instructions for saving and restoring part or all of the processor state before and after subroutine calls. Some kinds of subroutine or return instructions will include some kinds of save and restore of the processor state.