BS85B12-3 HOLTEK [Holtek Semiconductor Inc], BS85B12-3 Datasheet - Page 24

• Manufacturer Part Number: BS85B12-3
• Description: Touch Key Flash Type 8-Bit MCU with LCD/LED Driver
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

Rev. 1.00
Program Counter
Stack
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.
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 will be inserted. Manipulating
the PCL register may cause program branching, so an extra cycle is needed to pre-fetch.
This is a special part of the memory which is used to save the contents of the Program Counter only.
The stack has multiple levels depending upon the device and is neither part of the data nor part of the
program space, and is neither readable nor writeable. The activated level is indexed by the Stack
Pointer, and is neither readable nor writeable. At a subroutine call or interrupt acknowledge signal, the
contents of the Program Counter are pushed onto the stack. At the end of a subroutine or an interrupt
routine, signaled by a return instruction, RET or RETI, the Program Counter is restored to its previous
value from the stack. After a device reset, the Stack Pointer will point to the top of the stack.
If the stack is full and an enabled interrupt takes place, the interrupt request flag will be recorded but
the acknowledge signal will be inhibited. When the Stack Pointer is decremented, by RET or RETI, the
interrupt will be serviced. This feature prevents stack overflow allowing the programmer to use the
structure more easily. However, when the stack is full, a CALL subroutine instruction can still be
executed which will result in a stack overflow. Precautions should be taken to avoid such cases which
might cause unpredictable program branching.
If the stack is overflow, the first Program Counter save in the stack will be lost.
BS85B12-3
BS85C20-3
Device
Touch Key Flash MCU with LCD/LED Driver
Stack Levels
4
8
18
BS85B12-3/BS85C20-3
February 1, 2011