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SAFC161PI Datasheet(PDF) 78 Page - Infineon Technologies AG |
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SAFC161PI Datasheet(HTML) 78 Page - Infineon Technologies AG |
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78 / 400 page  &3, 7KH &HQWUDO 3URFHVVLQJ 8QLW &38 User’s Manual 4-30 1999-08 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 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 F000 H to FFFE H. 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 “System Programming”. 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 (FE16 H/0BH) Reset value: FC00 H 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 11 11 stkun 0 r r r r rw r Bit Function stkun Modifiable portion of register STKUN Specifies the upper limit of the internal system stack. |
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