ST92F124 datasheet(39/426 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - DEVICE ARCHITECTURE

Note: Setting the HIMP bit is recommended for

noise reduction when only internal Memory is

used.

If the memory access ports are declared as an ad-

dress AND as an I/O port (for example: P10... P14

= Address, and P15... P17 = I/O), the HIMP bit has

no effect on the I/O lines.

2.3.6 Stack Pointers

Two separate, double-register stack pointers are

available: the System Stack Pointer and the User

Stack Pointer, both of which can address registers

or memory.

The stack pointers point to the “bottom” of the

stacks which are filled using the push commands

and emptied using the pop commands. The stack

pointer is automatically pre-decremented when

data is “pushed” in and post-incremented when

data is “popped” out.

The push and pop commands used to manage the

System Stack may be addressed to the User

Stack by adding the suffix “u”. To use a stack in-

struction for a word, the suffix “w” is added. These

suffixes may be combined.

When bytes (or words) are “popped” out from a

stack, the contents of the stack locations are un-

changed until fresh data is loaded. Thus, when

data is “popped” from a stack area, the stack con-

tents remain unchanged.

Note: Instructions such as: pushuw

RR236

pushw

RR238,

as well as the corresponding

pop

instructions (where R236 & R237, and R238

& R239 are themselves the user and system stack

pointers respectively), must not be used, since the

pointer

values

are

themselves

automatically

changed by the push or pop instruction, thus cor-

rupting their value.

System Stack

The System Stack is used for the temporary stor-

age of system and/or control data, such as the

Flag register and the Program counter.

The following automatically push data onto the

System Stack:

– Interrupts

When entering an interrupt, the PC and the Flag

ENCSR bit in the EMR2 register is set, then the

Code Segment Register is also pushed onto the

System Stack.

– Subroutine Calls

When a call instruction is executed, only the PC

is pushed onto stack, whereas when a calls in-

struction (call segment) is executed, both the PC

and the Code Segment Register are pushed onto

the System Stack.

– Link Instruction

The link or linku instructions create a C lan-

guage stack frame of user-defined length in the

System or User Stack.

All of the above conditions are associated with

their counterparts, such as return instructions,

which pop the stored data items off the stack.

User Stack

The User Stack provides a totally user-controlled

stacking area.

The User Stack Pointer consists of two registers,

R236 and R237, which are both used for address-

ing a stack in memory. When stacking in the Reg-

ister File, the User Stack Pointer High Register,

R236, becomes redundant but must be consid-

ered as reserved.

Stack Pointers

Both System and User stacks are pointed to by

double-byte stack pointers. Stacks may be set up

in RAM or in the Register File. Only the lower byte

will be required if the stack is in the Register File.

The upper byte must then be considered as re-

served and must not be used as a general purpose

The stack pointer registers are located in the Sys-

tem Group of the Register File, this is illustrated in

Table 6.

Stack Location

Care is necessary when managing stacks as there

is no limit to stack sizes apart from the bottom of

any address space in which the stack is placed.

Consequently programmers are advised to use a

stack pointer value as high as possible, particular-

ly when using the Register File as a stacking area.

Group D is a good location for a stack in the Reg-

ister File, since it is the highest available area. The

stacks may be located anywhere in the first 14

groups of the Register File (internal stacks) or in

RAM (external stacks).

Note. Stacks must not be located in the Paged

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