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APPENDIX B – GLOSSARY
debugging program.
byte
A set of exactly eight binary bits.
—C—
C
An abbreviation for carry/borrow in the
condition codes register of the
MC68HC908AB32. When adding two
unsigned 8-bit numbers, the C bit is set if
the result is greater than 255 ($FF).
CCR
An abbreviation for condition code
register in the MC68HC908AB32. The
CCR has five bits (H, I, N, Z, and C) that
can be used to control conditional branch
instructions. The values of the bits in the
CCR are determined by the results of
previous operations. For example, after a
load accumulator (LDA) instruction, Z
will be set if the loaded value was $00.
clock
A square wave signal that is used to
sequence events in a computer.
command set
The command set of a CPU is the set of
all operations that the CPU knows how to
perform. One way to represent an
instruction set is with a set of shorthand
mnemonics such as LDA meaning load
A. Another representation of an
instruction set is the opcodes that are
recognized by the CPU.
condition codes register
The CCR has five bits (H, I, N, Z, and C)
that can be used to control conditional
branch commands. The values of the bits
in the CCR are determined by the results
of previous operations. For example, after
B-2
a load accumulator (LDA) instruction, Z
will be set if the loaded value was $00.
CPU
Central processor unit. The part of a
computer that controls execution of
instructions.
CPU cycles
A CPU clock cycle is one period of the
internal bus-rate clock. Normally, this
clock is derived by dividing a crystal
oscillator source by two or more so the
high and low times will be equal. The
length of time required to execute an
instruction is measured in CPU clock
cycles.
CPU registers
Memory locations that are wired directly
into the CPU logic instead of being part of
the addressable memory map. The CPU
always has direct access to the
information in these registers. The CPU
registers in an MC68HC908 are A (8-bit
accumulator), X (8-bit index register),
CCR (condition code register containing
the H, I, N, Z, and C bits), SP (stack
pointer), and PC (program counter).
cycles
See CPU cycles
—D—
data bus
A set of conductors that are used to
convey binary information from a CPU to
a memory location or from a memory
location to a CPU; in the
MC68HC908AB32, the data bus is 8-bits.
development tools
Software or hardware devices used to
develop computer programs and
M68ICS08ABUM/D
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