M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Computer Numbers and Codes
Instruction Mnemonics and Assemblers
Octal
Freescale Semiconductor, Inc.
An opcode such as $4C is understood by the CPU, but it is not very
meaningful to a human. To solve this problem, a system of mnemonic
instruction equivalents is used. The $4C opcode corresponds to the
INCA mnemonic, which is read “increment accumulator.” Although there
is printed information to show the correlation between mnemonic
instructions and the opcodes they represent, this information is seldom
used by a programmer because the translation process is handled
automatically by a separate computer program called an assembler.An
assembler is a program that converts a program written in mnemonics
into a list of machine codes (opcodes and other information) that can
be used by a CPU.
An engineer develops a set of instructions for the computer in mnemonic
form and then uses an assembler to translate these instructions into
opcodes that the CPU can understand. We will discuss instructions,
writing programs, and assemblers in other chapters. However, you
should understand now that people prepare instructions for a computer
in mnemonic form, but the computer understands only opcodes; thus, a
translation step is required to change the mnemonics to opcodes, and
this is the function of the assembler.
Before leaving this discussion of number systems and codes, we will
look at two additional codes you may have heard about. Octal (base 8)
notation was used for some early computer work but is seldom used
today. Octal notation used the numbers 0 through 7 to represent sets of
three binary digits in the same way hexadecimal is used to represent
sets of four binary digits. The octal system had the advantage of using
customary number symbols, unlike the hexadecimal symbols A through
F discussed earlier.
Two disadvantages caused octal to be abandoned for the hexadecimal
notation used today. First of all, most computers use 4, 8, 16, or 32 bits
per word; these words do not break down evenly into sets of three bits.
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
32 Computer Numbers and Codes
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