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S3C9228/P9228 ADDRESS SPACES
2-5
SYSTEM STACK
S3C9-series microcontrollers use the system stack for subroutine calls and returns and to store data. The PUSH
and POP instructions are used to control system stack operations. The S3C9228/P9228 architecture supports
stack operations in the internal register file.
STACK OPERATIONS
Return addresses for procedure calls and interrupts and data are stored on the stack. The contents of the PC are
saved to stack by a CALL instruction and restored by the RET instruction. When an interrupt occurs, the contents
of the PC and the FLAGS register are pushed to the stack. The IRET instruction then pops these values back to
their original locations. The stack address is always decremented before a push operation and incremented after
a pop operation. The stack pointer (SP) always points to the stack frame stored on the top of the stack, as shown
in Figure 2-4.
Stack contents
after a call
instruction
Stack contents
after an
interrupt
Top of
stack
Flags
PCH
PCL
PCL
PCH
Top of
stack
Low Address
High Address
Figure 2-4. Stack Operations
STACK POINTER (SP)
Register location D9H contains the 8-bit stack pointer (SP) that is used for system stack operations. After a reset,
the SP value is undetermined.
Because only internal memory space is implemented in the S3C9228/P9228, the SP must be initialized to an 8-
bit value in the range 00H–B7H.
NOTE
In case a Stack Pointer is initialized to 00H, it is decreased to FFH when stack operation starts. This
means that a Stack Pointer access invalid stack area.