[KERNAL] refactor, comment 65C816 native stack API

kernal/x16/65c816/stack.s

@@ -8,9 +8,10 @@
 .setcpu "65816"
 
 .segment "KRAM816S"
-stack_counter: .res 1
-stack_ptr: .res 1
-stack_one: .res 1
+stack_counter: .res 1 ; depth of the stack chain
+stack_ptr: .res 1     ; low byte of the preserved $01xx stack
+stack_one: .res 1     ; holds the value #$01 to speed up 16-bit
+                      ; stack restore operations
 
 .segment "MACHINE"
 
@@ -25,76 +26,110 @@ stack_one: .res 1
 .A16
 .I16
 
+;--------------------------------------------------------------
+; stack_enter_kernal_stack
+;
+; Function: change SP to KERNAL (page $01) SP
+; Flags:    m = 0, x = 0, e = 0
+; Input:    SP = old stack pointer
+; Output:   SP = KERNAL stack pointer (after rts)
+;--------------------------------------------------------------
+.proc stack_enter_kernal_stack
+	.A16
+	.I16
+	ldx stack_ptr
+	; fall through
+.endproc
+
 ;--------------------------------------------------------------
 ; stack_push
 ;
-; Function: 
-; Flags:    M = 0
-; Output:   
+; Function: change SP to new pointer, preserving the old one
+;           at the top of the new stack
+; Flags:    m = 0, x = 0, e = 0
+; Input:    X = new stack pointer, SP = old stack pointer
+; Output:   SP = X - 2 (after rts)
 ;--------------------------------------------------------------
 .proc stack_push
 	.A16
 	.I16
-	ply
-	lda stack_counter
-	beq @counter_zero
-
 	sei
-
+	ply                ; hold the return address
+	lda stack_counter  ; is the stack chain empty
+	beq @counter_zero  ; if so, don't push the old stack
+	                   ; instead, treat the old stack
+	                   ; as $01xx page, and preserve its location
 	inc
-	sta stack_counter
+	sta stack_counter  ; otherwise, increment the stack_counter
 
-	tsc
-	txs
-	pha
-	phy
+	tsc                ; preserve old SP
+	txs                ; bring new SP into effect
+	pha                ; save old SP on new stack
+	phy                ; restore the return address
 
 	cli
 	rts
 
 @counter_zero:
-	tsc
-	xba
-	sta stack_counter
-	txs
-	phy
+	tsc                ; This is inferred to be $01xx
+	xba                ; Swap bytes, $xx01
+	sta stack_counter  ; Store $01 to stack_counter
+	                   ; and $xx to stack_ptr (stack_counter+1)
+	txs                ; bring new SP into effect
+	phy                ; restore return address
+	cli
 	rts
 .endproc
 
-.proc stack_pop
+;--------------------------------------------------------------
+; stack_leave_kernal_stack
+;
+; Function: restore old SP, which is assumed to be in page $01
+; Flags:    m = 0, x = 0, e = 0
+; Input:    (SP+2) = old stack pointer if stack_counter > 1
+; Output:   SP = old stack pointer (after rts)
+;--------------------------------------------------------------
+.proc stack_leave_kernal_stack
 	.A16
 	.I16
-	ply
-	lda stack_counter
-	dec
-	bit #$00FF
-	beq @counter_one
+	; fall through
+.endproc
 
+;--------------------------------------------------------------
+; stack_pop
+;
+; Function: change SP to old pointer
+; Flags:    m = 0, x = 0, e = 0
+; Input:    (SP+2) = old stack pointer if stack_counter > 1
+; Output:   SP = old stack pointer (after rts)
+;--------------------------------------------------------------
+.proc stack_pop
+	.A16
+	.I16
 	sei
-	sta stack_counter
-	pla
-	tcs
-	phy
+	ply                ; hold the return address
+	lda stack_counter  ; C = $xxyy where $yy = stack_counter
+	                   ; and $xx = stack_ptr, the KERNAL
+	                   ; stack pointer
+	dec                ; decrement $xxyy
+	bit #$00FF         ; if $yy = 0
+	beq @counter_one   ; then restore SP to $01xx
+
+	sta stack_counter  ; store decremented stack_counter
+	pla                ; pop old SP off existing stack
+	tcs                ; bring old SP into effect
+	phy                ; restore return address
+	cli
 	rts
 
 @counter_one:
-	stz stack_counter
-	inc
-	xba
-	tcs
-	phy
+	stz stack_counter  ; reset stack_counter to zero
+	                   ; this is the end of the stack popping chain
+	inc                ; C = $xx00 -> $xx01
+	xba                ; C = $01xx (valid KERNAL SP)
+	tcs                ; bring KERNAL SP into effect
+	phy                ; restore return address
+	cli
 	rts
 .endproc
 
-.proc stack_enter_kernal_stack
-	.A16
-	.I16
-	lda stack_ptr
-	jmp stack_push
-.endproc
-
-.proc stack_leave_kernal_stack
-	.A16
-	.I16
-	jmp stack_pop
-.endproc