S3F82NB datasheet(39/361 Pages) ZILOG

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PS031601-0813

P R E L I M I N A R Y

S3F82NB

Product Specification

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 S3F82NB 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 2-16.

Stack contents

after a call

instruction

Stack contents

after an

interrupt

Top of

stack

Flags

PCH

PCL

PCH

Top of

stack

Low Address

High Address

Figure 2-16. 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)

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 S3F82NB, 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

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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