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ADDRESS SPACE
SYSTEM AND USER STACKS
S3C8-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 S3F80P5 architecture supports stack
operations in the internal register file.
Stack Operations
Return addresses for procedure calls, 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 registers are pushed to the stack. The IRET instruction then pops these values back to
their original locations. The stack address value is always decreased by one before a push operation and
increased by one 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 2-15.
Top of
stack
User-Defined Stacks
You can freely define stacks in the internal register file as data storage locations. The instructions PUSHUI,
PUSHUD, POPUI, and POPUD support user-defined stack operations.
Stack Pointers (SPL)
Register location D9H contains the 8-bit stack pointer (SPL) that is used for system stack operations. After a
reset, the SPL value is undetermined. Because only internal memory 256-byte is implemented in The S3F80P5,
the SPL must be initialized to an 8-bit value in the range 00–FFH.
2-20
High Address
PCL
PCH
Stack contents
after a call
instruction
Low Address
Figure 2-16. Stack Operations
PCL
PCH
Top of
Flags
stack
Stack contents
after an
interrupt
S3F80P5_UM_ REV1.00
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