ST9291 datasheet(16/20 Pages) STMICROELECTRONICS

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SYSTEM REGISTERS (Continued)

1.1.3.6 Stack Pointers

There are two separate, double register stack

pointers available (named System Stack Pointer

and User Stack Pointer), both of which can ad-

dress 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”.

For example, the register address space is se-

lected for a stack and the corresponding stack

pointer register contains 220. When a byte of data

is “pushed” into the stack, the stack pointer register

is decremented to 219, then the data byte is

“loaded” into register 219. Conversely, if a stack

pointer register contains 189 and a byte of data is

“popped” out, the byte of data is then extracted

from the stack and then the stack pointer register is

incremented to 190.

The push and pop commands used to manage

the system stack area are made applicable to the

user stack by adding the suffix U, while to use a

stack instruction for a word a W is added.

For example push inserts data into the system

stack, but an added U indicates the user stack and

W means a word, so the instruction pushuw loads

a word into the bottom of the user stack.

If the User Stack Pointer register contains 223

(working in register space) the instruction pushuw

will decrement User Stack Pointer register to 222

and then load a word into register R222 and R221.

When bytes (or words) are “popped out” the values

in those registers are left unchanged until fresh

data is loaded into those locations. Thus when

data is “popped” out from a stack area, the stack

content remains unchanged.

Note. Stacks must not be located in the pages or

the system register area.

The System Stack area and The System Stack

Pointer

The System Stack area is used for the storage of

temporarily suspended system and/or control reg-

isters, i.e. the Flag register and the Program

counter, while interrupts are being serviced. For

subroutine execution only the Program Counter

needs to be saved in the System stack area.

There are two situations when this occurs automat-

ically, one being when an interrupt occurs and the

other when the instruction call subroutine is used.

When the system stack area is in the Register File,

the stack pointer, which points to the bottom of the

stack, only needs one byte for addressing, in which

case the System Stack Pointer Low Register

(R239) is sufficient for addressing purposes. As a

result the System Stack Pointer High Register

(R238) becomes redundant BUT must be consid-

ered as reserved (please refer also to “spurious”

memory access section). Clearly when the stack is

external a full word address is necessary and so

both registers are used to point, the even register

providing the MSB and the odd register providing

the LSB.

The User Stack area and User Stack Pointer

The User Stack area is completely free from all in-

terference from automatic operations and so it pro-

vides a totally user controlled stacking area, that

area being in any part of the memory which is of a

RAM nature, or the first 14 groups of the general

Paged group.

The User Stack Pointer consists of two registers,

R236 and R237, which are both used for address-

ing an external stack, while, when stacking in the

ter, R236, becomes redundant but must be consid-

ered as reserved.

ST9291

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