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cc65/libsrc/runtime/stkchk.s

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;
; Ullrich von Bassewitz, 19.03.2001
;
; Stack checking code.
; For performance reasons (to avoid having to pass a parameter), the compiler
; calls the stkchk routine *after* allocating space on the stack. So the
; stackpointer may already be invalid if this routine is called. In addition
; to that, pushs and pops that are needed for expression evaluation are not
; checked (this would be way too much overhead). As a consequence we will
; operate using a safety area at the stack bottom. Once the stack reaches this
; safety area, we consider it an overflow, even if the stack is still inside
; its' bounds.
;
.export stkchk
.constructor initstkchk, 25
.import __STACKSIZE__ ; Linker defined
.import pusha0, _exit
.importzp sp
; Use macros for better readability
.macpack generic
.code
; ----------------------------------------------------------------------------
; Initialization code. This is a constructor, so it is called on startup if
; the linker has detected references to this module.
.proc initstkchk
lda sp
sta initialsp
sub #<__STACKSIZE__
sta lowwater
lda sp+1
sta initialsp+1
sbc #>__STACKSIZE__
add #1 ; Add 256 bytes safety area
sta lowwater+1
rts
.endproc
; ----------------------------------------------------------------------------
; Stack checking routine. Does not need to save any registers.
.proc stkchk
lda lowwater+1
cmp sp+1
bcs @L1
rts
; Check low byte
@L1: bne @Overflow
lda lowwater
cmp sp
bcs @Overflow
rts
; We have a stack overflow. Set the stack pointer to the initial value, so
; we can continue without worrying about stack issues.
@Overflow:
lda initialsp
sta sp
lda initialsp+1
sta sp+1
lda #4
jsr pusha0
jmp _exit
.endproc
; ----------------------------------------------------------------------------
; Data
.bss
; Initial stack pointer value. Stack is reset to this in case of overflows to
; allow program exit processing.
initialsp: .word 0
; Stack low water mark.
lowwater: .word 0