M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Computer Numbers and Codes
ASCII Code
Computer Operation Codes
Freescale Semiconductor, Inc.
Computers must handle many kinds of information other than just
numbers. Text (alphanumeric characters) and instructions must be
encoded in such a way that the computer can understand this
information. The most common code for text information is the American
Standard Code for Information Interchange (or ASCII). The ASCII code
establishes a widely accepted correlation between alphanumeric
characters and specific binary values. Using the ASCII code, $41
corresponds to capital A, $20 corresponds to a space character, etc. The
ASCII code translates characters to 7-bit binary codes, but in practice
the information is most often conveyed as 8-bit characters with the most
significant bit equal to 0. This standard code allows equipment made by
various manufacturers to communicate because all of the machines use
this same code.
Table 2 shows the relationship between ASCII characters and
hexadecimal values.
Computers use another code to give instructions to the CPU. This code
is called an operation code or opcode. Each opcode instructs the CPU
to execute a very specific sequence of steps that together accomplish an
intended operation. Computers from different manufacturers use
different sets of opcodes because these opcodes are internally
hard-wired in the CPU logic. The instruction set for a specific CPU is
the set of all operations that the CPU knows how to perform. Opcodes
are one representation of the instruction set and mnemonics are
another. Even though the opcodes differ from one computer to another,
all digital binary computers perform the same kinds of basic tasks in
similar ways. For instance, the CPU in the MC68HC05 MCU can
understand 62 basic instructions. Some of these basic instructions have
several slight variations, each requiring a separate opcode. The
instruction set of the MC68HC05 is represented by 210 unique
instruction opcodes. We will discuss how the CPU actually executes
instructions in another chapter, but first we need to understand a few
more basic concepts.
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
30 Computer Numbers and Codes
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