Chapter 2
Instruction Set Introduction
Instruction Set
Central to the successful operation of any microcontroller is its instruction set, which is a set of pro
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gram instruction codes that directs the microcontroller to perform certain operations. In the case of
Holtek microcontrollers, a comprehensive and flexible set of over 60 instructions is provided to en
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able programmers to implement their application with the minimum of programming overheads.
For easier understanding of the various instruction codes, they have been subdivided into several
functional groupings.
Instruction Timing
Most instructions are implemented within one instruction cycle. The exceptions to this are branch,
call, or table read instructions where two instruction cycles are required. One instruction cycle is
equal to 4 system clock cycles, therefore in the case of an 8MHz system oscillator, most instruc-
tions would be implemented within 0.5ms and branch or call instructions would be implemented
within 1ms. Although instructions which require one more cycle to implement are generally limited
to the JMP, CALL, RET, RETI and table read instructions, it is important to realize that any other in-
structions which involve manipulation of the Program Counter Low register or PCL will also take
one more cycle to implement. As instructions which change the contents of the PCL will imply a di-
rect jump to that new address, one more cycle will be required. Examples of such instructions
would be ²CLR PCL² or ²MOV PCL, A². For the case of skip instructions, it must be noted that if
the result of the comparison involves a skip operation then this will also take one more cycle, if no
skip is involved then only one cycle is required.
Moving and Transferring Data
The transfer of data within the microcontroller program is one of the most frequently used opera
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tions. Making use of three kinds of MOV instructions, data can be transferred from registers to the
Accumulator and vice-versa as well as being able to move specific immediate data directly into the
Accumulator. One of the most important data transfer applications is to receive data from the input
ports and transfer data to the output ports.
Chapter 2 Instruction Set Introduction
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