Samsung S3C84E5 System and User Stack, Stack Operations, Stack Pointers SPL, SPH, High Address

S3C84E5/C84E9/P84E9 ADDRESS SPACES

SYSTEM AND USER STACK

The S3C8-series microcontrollers use the system stack for data storage, subroutine calls and returns. The PUSH and POP instructions are used to control system stack operations. The S3C84E5/C84E9/P84E 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 register 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

PCL

PCH

Stack contents after a call instruction

High Address

Top of

stack

Low Address

PCL

PCH

Flags

Stack contents

after an interrupt

Figure 2-15. Stack Operations

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, SPH)

Register locations D8H and D9H contain the 16-bit stack pointer (SP) that is used for system stack operations. The most significant byte of the SP address, SP15–SP8, is stored in the SPH register (D8H), and the least significant byte, SP7–SP0, is stored in the SPL register (D9H). After a reset, the SP value is undetermined.

Because only internal memory space is implemented in the S3C84E5/C84E9/P84E, the SPL must be initialized to an 8-bit value in the range 00H–FFH. The SPH register is not needed and can be used as a general-purpose register, if necessary.

When the SPL register contains the only stack pointer value (that is, when it points to a system stack in the register file), you can use the SPH register as a general-purpose data register. However, if an overflow or underflow condition occurs as a result of increasing or decreasing the stack address value in the SPL register during normal stack operations, the value in the SPL register will overflow (or underflow) to the SPH register, overwriting any other data that is currently stored there. To avoid overwriting data in the SPH register, you can initialize the SPL value to "FFH" instead of "00H".

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