13
8Bit Single Chip Microcontroller
DMC73C168
4.4 Stack Pointer (SP)
The Stack Pointer(SP) is an 8-bit register in the CPU that is typically used to hold a pointer in RAM
(the Register File). However, the SP can also be used as temporary data storage if a stack is not
implemented, or if the SP contents are not needed. When a stack is implemented just before data is
pushed onto the stack and automatically decremented immediately after data is poped from the
stack. Upon assertion if the RESET function (see Section 4.7) >01 is loaded into the SP. The size
of the stack can be changed from the 254-level stack at RESET to a smaller stack by executing
a stack initialization program as illustrated in Figure 4.4. The This feature allows the stack to be
located anywhere in the Register File. The SP is initialized through the B register (R1).
RF
RF
RF
SP
>0001
>0002
>0003
>0004
>0005
>0006
>01
ST
Interrupt
SP
PCH
PCL
>04
SP
PCH
PCL
CALL
>06
Figure 4.4 Example of Stack Initialization in the Register File
4.5 Status Register (ST)
The Status Register (ST) is an 8-bit register in the CPU that contains three conditional status bits ;
Carry (C), Sign (N), Zero (Z), and a global Interrupt Enable bit (I) as shown in Figure 4-5.
Bit
7
6
5
Z
4
I
3
2
1
0
C
N
FUTURE USE
C : CARRY OUT
N : SIGN
Z : ZERO
I
: INTERRUPT ENABLE
Figure 4-5. Status Register (ST)
The C, N and Z bits are used mostly for arithmetic operations, bit rotating, and conditional branching.
The Carry(C) bits is used as the carry-in and carry-out for most of lotate and arithmetic instructions.
The Sign(N) bit contains the most significant bit of the destination operand contents after instruction
execution. The Zero(Z) bit contains a one when all bits of the destination operand are equal to zero
after instruction execution. The C, N and Z status bits also have jump-on-condition instructions
associated them. The global Interrupt Enable (I) bit must be set to one by the EINT instruction in
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