Program Counter - Holtek BS86DH12C Manual

f
SYS
(System Clock)
Phase Clock T1
Phase Clock T2
Phase Clock T3
Phase Clock T4

Program Counter

Pipelining
For instructions involving branches, such as jump or call instructions, two machine cycles are
required to complete instruction execution. An extra cycle is required as the program takes one
cycle to first obtain the actual jump or call address and then another cycle to actually execute the
branch. The requirement for this extra cycle should be taken into account by programmers in timing
sensitive applications.
1 MOV A,[12H]
2 CALL DELAY
3 CPL [12H]
4 :
5 :
6 DELAY: NOP
Program Counter
During program execution, the Program Counter is used to keep track of the address of the next
instruction to be executed. It is automatically incremented by one each time an instruction is executed
except for instructions, such as "JMP" or "CALL" that demand a jump to a non-consecutive Program
Memory address. Only the lower 8 bits, known as the Program Counter Low Register, are directly
addressable by the application program.
When executing instructions requiring jumps to non-consecutive addresses such as a jump instruction,
a subroutine call, interrupt or reset, etc., the microcontroller manages program control by loading
the required address into the Program Counter. For conditional skip instructions, once the condition
has been met, the next instruction, which has already been fetched during the present instruction
execution, is discarded and a dummy cycle takes its place while the correct instruction is obtained.
The lower byte of the Program Counter, known as the Program Counter Low register or PCL, is
available for program control and is a readable and writeable register. By transferring data directly
into this register, a short program jump can be executed directly; however, as only this low byte
is available for manipulation, the jumps are limited to the present page of memory that is 256
locations. When such program jumps are executed it should also be noted that a dummy cycle
Rev. 1.00
High Voltage Touch A/D Flash MCU with HVIO
PC
Fetch Inst. (PC)
Execute Inst. (PC-1) Fetch Inst. (PC+1)
Execute Inst. (PC)
System Clocking and Pipelining
Fetch Inst. 1 Execute Inst. 1
Fetch Inst. 2
Instruction Fetching
Program Counter
High Byte
PC12~PC8
Program Counter
26
BS86DH12C
PC+1 PC+2
Fetch Inst. (PC+2)
Execute Inst. (PC+1)
Execute Inst. 2
Fetch Inst. 3 Flush Pipeline
Fetch Inst. 6 Execute Inst. 6
Low Byte (PCL)
PCL7~PCL0
October 26, 2018
Fetch Inst. 7