[KERNAL] add extapi16 #0 test reflector call

kernal/x16/65c816/extapi16.s

@@ -1,3 +1,7 @@
+; Comments in this source file use this nomenclature
+; For registers, a dot and capital letters, for example: .X .Y .A .P .C
+; For processor flags, a lowercase letter, for example: x m c z n
+
 .import stack_push, stack_pop
 .import stack_enter_kernal_stack, stack_leave_kernal_stack
 
@@ -12,31 +16,43 @@
 .A16
 .I16
 
-; This API call expects and requires m=0, x=0, e=0
+; This API call expects and requires m=0, e=0
+; Some calls require x=0, some allow x=1
 extapi16:
-    php ; preserve flags
-    set_carry_if_65c816
-    bcc unsupported
-    phx ; preserve X parameter
-    asl ; translate API call number to jump table entry
-    tax
-    lda apitbl,x
-    plx ; restore old X
-    plp ; restore flags
-    pha ; push api address-1 onto stack
-    rts ; jump to api
+	php ; preserve flags
+	set_carry_if_65c816
+	bcc unsupported
+	phx ; preserve X parameter
+	asl ; translate API call number to jump table entry
+	tax
+	lda apitbl,x
+	plx ; restore old X
+	plp ; restore flags
+	pha ; push api address-1 onto stack
+	rts ; jump to api
 
 unsupported:
-    plp
+	plp
 secrts:
-    sec
-    rts
+	sec
+	rts
 
 apitbl:
-    .word secrts-1 ; slot 0 is reserved
-    .word stack_push-1                 ; API 1
-    .word stack_pop-1                  ; API 2
-    .word stack_enter_kernal_stack-1   ; API 3
-    .word stack_leave_kernal_stack-1   ; API 4
-
-
+	.word addition_test-1              ; API 0 (x=0 or x=1)
+	.word stack_push-1                 ; API 1 (x=0)
+	.word stack_pop-1                  ; API 2 (x=0)
+	.word stack_enter_kernal_stack-1   ; API 3 (x=0)
+	.word stack_leave_kernal_stack-1   ; API 4 (x=0)
+
+
+addition_test: ; add .X to .Y, no carry, return in .C, used in the jsrfar unit tests
+	php
+	rep #$31
+	.A16
+	.I16
+	phx
+	tya
+	adc $01,S
+	plx
+	plp
+	rts