TastierProjectFooter.s

; Subroutine enter
; Construct stack frame for procedure
; Input: R0 - lexic level (LL)
;		 R1 - number of local variables
; Output: new stack frame

enter
	STR		R0, [TOP,#4]			; set lexic level
	STR		BP, [TOP,#12]			; and dynamic link
	; if called procedure is at the same lexic level as
	; calling procedure then its static link is a copy of
	; the calling procedure's static link, otherwise called
 	; procedure's static link is a copy of the static link 
	; found LL delta levels down the static link chain
    LDR		R2, [BP,#4]				; check if called LL (R0) and
	SUBS	R0, R2					; calling LL (R2) are the same
	BGT		enter1
	LDR		R0, [BP,#8]				; store calling procedure's static
	STR		R0, [TOP,#8]			; link in called procedure's frame
	B		enter2
enter1
	MOV		R3, BP					; load current base pointer
	SUBS	R0, R0, #1				; and step down static link chain
    BEQ     enter2-4                ; until LL delta has been reduced
	LDR		R3, [R3,#8]				; to zero
	B		enter1+4				;
	STR		R3, [TOP,#8]			; store computed static link
enter2
	MOV		BP, TOP					; reset base and top registers to
	ADD		TOP, TOP, #16			; point to new stack frame adding
	ADD		TOP, TOP, R1, LSL #2	; four bytes per local variable
	BX		LR						; return
	
	AREA	Memory, DATA, READWRITE
globals     SPACE 4096
stack      	SPACE 16384

	END