Machine language





A system of codes directly understandable by a computer's CPU is termed this
CPU's native or machine language. Although machine code may seem similar to
assembly language they are in fact two different types of languages.
Assembly code consists of both binary numbers and simple words whereas
machine code is composed only of the two binary digits 0 and 1. Every CPU
has its own machine language, although there is considerable overlap between
some. If CPU A understands the full language of CPU B it is said that A is
compatible with B. CPU A may not be compatible with CPU B, as A may know a
few codes that B does not.

The "words" of a machine language are called instructions; each of these
gives a basic command to the CPU. A program is just a long list of
instructions that are executed by a CPU. Normally instructions are executed
one after the other, but program flow may be influenced by special jump
instructions that transfer execution to an instruction other than the
following one. Conditional jumps are taken (execution continues at another
address) or not (execution continues at the next instruction) depending on
some condition.

Instructions are simply a pattern of bits -- different patterns correspond
to different commands to the machine. The more readable rendition of a
machine language is called assembly language.

Some languages give all their instructions the same number of bits, while
the instruction length differs in others. How the patterns are organised
depends largely on the specific language. Common to most is the division of
an instruction into fields, of which one or more specify the exact operation
(for example "add"). Other fields may give the type of the operands, their
location, or their value directly (operands contained in an instruction are
called immediate).

As a specific example, let us take a look at the MIPS architecture. Its
instructions are always 32 bit long. The general type of instruction is
given by the op field, the highest 6 bits. J-type and I-type instructions
are fully specified by op. R-type instructions include an addtional field
funct to determine the exact operation. The fields used in these types are:

    6      5     5     5     5      6 bits
 [  op  |  rs |  rt |  rd |shamt| funct]  R-type
 [  op  |  rs |  rt | address/immediate]  I-type
 [  op  |        target address        ]  J-type

rs, rt, and rd indicate register operands; shamt gives a shift amount; and
the address or immediate fields contain an operand directly.

For example adding the registers 1 and 2 and placing the result in register
6 is encoded:

 [  op  |  rs |  rt |  rd |shamt| funct]
     0     1     2     6     0     32     decimal
  000000 00001 00010 00110 00000 100000   binary

Loading a value from the memory cell 68 cells after the one register 3
points to into register 8:

 [  op  |  rs |  rt | address/immediate]
    35     3     8           68           decimal
  100011 00011 01000 000000000001000100   binary

Jumping to the address 1025:

 [  op  |        target address        ]
     2                 1025               decimal
  000010 000000000000000000010000000001   binary