Instruction Set
Introduction
Central to the successful operation of any microcontroller is its instruction set, which is a set of
program 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 enable
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 instructions would be
implemented within 0.5
m
s and branch or call instructions would be implemented within 1
m
s. 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 instructions 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 direct jump to that new address, one
more cycle will be required. Examples of such instructions would be
²
CLR PCL
²
or
²
MOV PCL, A
²
.
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 operations.
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.
Arithmetic Operations
The ability to perform certain arithmetic operations and data manipulation is a necessary feature of
most microcontroller applications. Within the Holtek microcontroller instruction set are a range of add
and subtract instruction mnemonics to enable the necessary arithmetic to be carried out. Care must be
taken to ensure correct handling of carry and borrow data when results exceed 255 for addition and less
than 0 for subtraction. The increment and decrement instructions INC, INCA, DEC and DECA
provide a simple means of increasing or decreasing by a value of one of the values in the destination
specified.
Logical and Rotate Operations
The standard logical operations such as AND, OR, XOR and CPL all have their own instruction within
the Holtek microcontroller instruction set. As with the case of most instructions involving data
manipulation, data must pass through the Accumulator which may involve additional programming
steps. In all logical data operations, the zero flag may be set if the result of the operation is zero.
Another form of logical data manipulation comes from the rotate instructions such as RR, RL, RRC
and RLC which provide a simple means of rotating one bit right or left. Different rotate instructions
exist depending on program requirements. Rotate instructions are useful for serial port programming
applications where data can be rotated from an internal register into the Carry bit from where it can be
examined and the necessary serial bit set high or low. Another application where rotate data operations
are used is to implement multiplication and division calculations.
Rev. 1.50
112
April 28, 2020
BS83B08-3/B12-3/B16-3/B16G-3/C24-3
8-Bit Touch Key Flash MCU
