SX Microcontroller Memory Method

Software Stacks

Yes, the nested interrupts are possible. The method I used processed two interrupts, but more interrupts could be okay. To do this, I use software stack macros (see attachment) that use FSR as a stack pointer. A typical interrupt handler looks like this:
	org	$004	; interrupt vector location
            PUSHW                             ;store W
            PUSHSTATUS                 ;store STATUS

                                            ;code goes here

            POPSTATUS                      ;restore W
            POPW                            ;restore STATUS
            reti	; return from interrupt
After storing W and STATUS goes usual interrupt flags polling .
As soon as a set flag is detected, it should be cleared to allow other interrupts to actually interrupt the process.
Be careful not to allow too many interrupts to be processed, only the ones that have critical response time, because hardware stack may overflow. The best way is to not clear the interrupt flag during the most critical interrupt so that it could finish without unwanted interrupts.
Priority of different interrupts is programmed by flags polling sequence(in case they trigger at the same time).
That's it.
By the way, I find software stack macros very useful in practically every program. The best example is when you need one scratch pad register. Using stack it's simple:
PUSH    ;reserve one byte on stack
....use INDF as a scratch pad reg
POP    ;restore stack
Good luck.
;*******************************
;Software stack organization
;*******************************
;FSR is a stack pointer
;Top of stack is in INDF
;Stack grows in upper addresses direction
;*******************************************************************
;MACRO:      INSTR.NUMBER:   STATUS INFLUENCE:          W INFLUENCE:
;
;PUSHW          2               no                      no
;POPW           3               no                      yes
;POPW2          2               yes!!!                  yes
;PUSHSTATUS     3               no                      yes!!!
;POPSTATUS      3               yes                     yes!!!
;PUSH           1               no                      no
;POP            1               no                      no
;*******************************************************************
;Notes: 1) FSR should point at the stack top
;       2) PUSHSTATUS and POPSTATUS must be used in pair,
;       because these macros mangle quadruples
;*******************************************************************

PUSHW   MACRO
	incsz	FSR	;never goes zero, but STATUS is safe
	mov	INDF, W
        ENDM

POPW    MACRO
	swap	INDF
	mov	W, <>INDF
	decsz	FSR
        ENDM

POPW2   MACRO
	mov	W, INDF
	decsz	FSR	;the popped W STATUS is safe
        ENDM

PUSHSTATUS      MACRO
	mov	W, <>STATUS
	incsz	FSR	;never goes zero, but STATUS is safe
	mov	INDF, W
        ENDM

POPSTATUS       MACRO
	mov	W, <>INDF
	mov	STATUS, W
	decsz	FSR
        ENDM

PUSH    MACRO
	incsz	FSR
        ENDM

POP     MACRO
	decsz	FSR
        ENDM

Robin Abbott of Forest Electronic Developments says:

This might be of use to someone. Recently I had a project where a subroutine took a value in W and saved to a software stack:
	mov	Temp, W
	mov	W, sp	; Stack pointer
	mov	FSR, W	; Point to it
	mov	W, Temp
	mov	Ind, W
Trouble is it uses a temporary variable which I didn't have (it is in an interrupt). This alternative which makes use of XOR uses no temporary variable at the expense of 1 extra word:
	mov	FSR, W
	mov	W, sp
	xor	FSR, W
	xor	W, FSR
	xor	FSR, W
	mov	Ind, W
You can also use this to swap two variables (say x and y) without a temporary variable leaving X (or Y if order is reversed) in W.
	mov	W, x	; Get X
	xor	y, W	; Y is now X^Y
	xor	W, y	; W is now (X^y)^X==Y  (say OldY)
	mov	x, W	; Now X is OldY
	xor	y, W	; finally Y is (OldX^Y)^Y==OldX
I think this may be an old technique - I have vague memories of something similar from the pre-history of programming, but only found a use for it now

Keep in mind, that on the SX 18..28, fsr.4 must be set after each decrement of fsr to prevent it from overwritting the global registers.

Paul Baker shares this code:

;-----------------------------------------------
; Constant Declarations
;-----------------------------------------------
;def _NOSMASH		;Unncomment to prevent stack over/underflows
			;you must provide routines DUnderflow and DOverflow to handle exception
			;and rptr which is maximum memory location
;def _SX52_		;no wrapping code and move vars up to make room for ports D and E
dstack		equ $10 ;initial head of data stack (this is ajusted upwards when direct store variables are declared)

;-----------------------------------------------
; Variable Declarations
;-----------------------------------------------
ifdef _SX52_
	org $0A
else
	org $08
endif
wlo		ds  1				;low byte of working word
whi		ds  1				;high byte of working word
dptr		ds  1				;pointer to stack

;-----------------------------------------------
;
; Stack Functions
;
;-----------------------------------------------
InitStack	clr  wlo			;clear working registers
		clr  whi
		mov  dptr, #dstack		;init pointer to data stack
		ret

;-----------------------------------------------
; Pushes contents of wlo onto data stack
; wlo -> stack+
; Affects: w, dptr, fsr
;-----------------------------------------------
DPush		mov  fsr,dptr			;load fsr with stack pointer
		mov  ind,wlo			;store byte on top of stack
		inc  dptr			;adjust stack pointer
ifndef _SX52_	
		setb dptr.4			;SX28 bank wrapping adjustment
endif
ifdef _NOSMASH
		cje  dptr, rptr, DOverflow	;stack over
endif
		retp

;-----------------------------------------------
; Pops top of data stack into wlo
; -stack -> wlo
; Affects: w, wlo, dptr, fsr
;-----------------------------------------------
DPop		dec  dptr			;adjust stack pointer
ifndef _SX52_
		jb   dptr.4, :noroll		;SX28 bank wrapping adjustment
		 sub dptr, #16
:noroll	
endif
ifdef _NOSMASH
		cjb  dptr, dstack, DUnderflow 	;stack under
endif
		mov  fsr, dptr			;load fsr with stack pointer
		mov  wlo, ind			;fetch byte from top of stack
		retp

;-----------------------------------------------
; Fetchs wth element of data stack into w, 
; only guaranteed to work with w<16 for SX28
; stack(w)->w
; Affects: w, fsr
;-----------------------------------------------
DPeek		mov  w,dptr-w			;adjust stack pointer by -w
ifdef _NOSMASH
		cjb  dptr, dstack, DUnderflow 	;stack under
endif
		mov  fsr, w			;load stack pointer into fsr
ifndef _SX52_
		jb   dptr.4, :noroll		;SX28 bank wrapping adjustment
		 sub dptr, #16
:noroll	
endif
		mov  w,ind			;fetch byte at stack pointer
		retp

;-----------------------------------------------
; Pushes wlo then whi onto data stack
; wlo,whi -> stack++
; Affects: w, wlo, fsr, dptr
;-----------------------------------------------
DPush2		call DPush
		mov  wlo, whi
		call DPush
		retp

;-----------------------------------------------
; Pops top of data stack into whi then wlo
; --stack -> wlo,whi <-top of stack
; Affects: w, wlo, whi, fsr, dptr
;-----------------------------------------------
DPop2		call DPop
		mov  whi, wlo
		call DPop
		retp