C167CS-4RM datasheet(79/517 Pages) INFINEON

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C167CS

Derivatives

The Central Processing Unit (CPU)

User’s Manual

4-29

V2.0, 2000-07

The Stack Underflow Pointer STKUN

This non-bit addressable register is compared against the SP register after each

operation, which pops data from the system stack (e.g. POP and RET instructions) and

after each addition to the SP register. If the content of the SP register is greater than the

content of the STKUN register, a stack underflow hardware trap will occur.

Since the least significant bit of register STKUN is tied to ‘0’ and bits 15 through 12 are

tied to ‘1’ by hardware, the STKUN register can only contain values from F000H to

FFFEH.

The Stack Underflow Trap (entered when (SP) > (STKUN)) may be used in two different

ways:

• Fatal error indication treats the stack underflow as a system error through the

associated trap service routine.

• Automatic system stack refilling allows to use the system stack as a ‘Stack Cache’

for a bigger external user stack. In this case register STKUN should be initialized to a

value, which represents the desired highest Bottom of Stack address.

More details about the stack underflow trap service routine and virtual stack

management are given in Chapter 22.

Scope of Stack Limit Control

The stack limit control realized by the register pair STKOV and STKUN detects cases

where the stack pointer SP is moved outside the defined stack area either by ADD or

SUB instructions or by PUSH or POP operations (explicit or implicit, i.e. CALL or RET

instructions).

This control mechanism is not triggered, i.e. no stack trap is generated, when

• the stack pointer SP is directly updated via MOV instructions

• the limits of the stack area (STKOV, STKUN) are changed, so that SP is outside of

the new limits.

STKUN

Stack Underflow Reg.

SFR (FE16H/0BH)

Reset Value: FC00H

98765

43210

1111

stkun

Bit

Function

stkun

Modifiable portion of register STKUN

Specifies the upper limit of the internal system stack.

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