Samsung KS57C2308 manual Stack Pointer SP, SP3 SP2 SP1, SP7 SP6 SP5 SP4, Sp,Ea

ADDRESS SPACES KS57C2308/P2308/C2316/P2316

STACK OPERATIONS

STACK POINTER (SP)

The stack pointer (SP) is an 8-bit register that stores the address used to access the stack, an area of data memory set aside for temporary storage of data and addresses. The SP can be read or written by 8-bit control instructions. When addressing the SP, bit 0 must always remain cleared to logic zero.

F80H	SP3	SP2	SP1	"0"
F81H	SP7	SP6	SP5	SP4

There are two basic stack operations: writing to the top of the stack (push), and reading from the top of the stack (pop). A push decrements the SP and a pop increments it so that the SP always points to the top address of the last data to be written to the stack.

The program counter contents and program status word are stored in the stack area prior to the execution of a CALL or a PUSH instruction, or during interrupt service routines. Stack operation is a LIFO (Last In-First Out) type. The stack area is located in general-purpose data memory bank 0.

During an interrupt or a subroutine, the PC value and the PSW are saved to the stack area. When the routine has completed, the stack pointer is referenced to restore the PC and PSW, and the next instruction is executed.

The SP can address stack registers in bank 0 (addresses 000H-0FFH) regardless of the current value of the enable memory bank (EMB) flag and the select memory bank (SMB) flag. Although general-purpose register areas can be used for stack operations, be careful to avoid data loss due to simultaneous use of the same register(s).

Since the RESET value of the stack pointer is not defined in firmware, we recommend that you initialize the stack pointer by program code to location 00H. This sets the first register of the stack area to 0FFH.

NOTE

A subroutine call occupies six nibbles in the stack; an interrupt requires six. When subroutine nesting or interrupt routines are used continuously, the stack area should be set in accordance with the maximum number of subroutine levels. To do this, estimate the number of nibbles that will be used for the subroutines or interrupts and set the stack area correspondingly.

+PROGRAMMING TIP — Initializing the Stack Pointer

To initialize the stack pointer (SP):

1. When EMB = "1":

SMB	15	; Select memory bank 15
LD	EA,#00H	; Bit 0 of SP is always cleared to "0"
LD	SP,EA	; Stack area initial address (0FFH) ← (SP) – 1
2. When EMB = "0":
LD	EA,#00H
LD	SP,EA	; Memory addressing area (00H–7FH, F80H–FFFH)

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