Samsung 8-Bit CMOS Microcontroller, S3C9228/P9228 user manual System Stack

S3C9228/P9228 ADDRESS SPACES

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.

Top of stack

PCL

PCH

Stack contents after a call instruction

High Address

Top of

stack

Low Address

PCL

PCH

Flags

Stack contents

after an interrupt

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.

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