OpenVMS Source-Code Demos

PEEK_ANY_INT_VAX.MAR

;--------------------------------------------------------------------------------
; Title  : Peek_any_int_vax.mar
; Author : Neil S. Rieck (Kitchener/Waterloo/Cambridge, Ontario, Canada)
; Created: 1999.11.13
;
; Notes:
; 1. This VAX assembly language function peeks into a memory location
; 2. It is tested for use with VAX-BASIC (see peek_demo2.bas)
; 3. parameters are passed here via register ap (argument pointer) a.k.a. r12
; 4. results are returned via r0 (and sometimes r1)
; 5. only 1, 2 or 4 bytes are supported (any other value returns 1 byte)
; 6. support could be easily added for 24 bits (RFA?) and 64 bits (quads)
; 7. since the VAX is a CISC machine, I used a CASE instruction (to show off)
;--------------------------------------------------------------------------------
	.entry peek_any_int_vax,^M<>	; function name is PEEK_ANY_INT_VAX (null entry mask)
	clrq    r0			; clear both r0 + r1 (cuz q = 64 bit)
	movl    @4(ap),r2		; passed parameter #1 is address of data
	movl	@8(ap),r3		; passed parameter #2 is desired number of bytes
					;
	casel	r3, #1, #3		; legal range is 1-4 (table has words 0-3)
CASETBL:
	.WORD	B1-CASETBL		; if r3 = 1 (then get 1 byte)
	.WORD	B2-CASETBL		; if r3 = 2 (then get 2 bytes)
	.WORD	B1-CASETBL		; if r3 = 3 (then get 1 bytes) cuz UNSUPPORTED
	.WORD	B4-CASETBL		; if r3 = 4 (then get 2 bytes)
	brw	B1-CASETBL		; ERROR if r3>4 or r3<1
B1:	movb    (r2),r0			; do an indirect byte move into r0
	brw	EXIT
B2:	movw	(r2),r0			; do an indirect word move into r0
	brw	EXIT
B4:	movl	(r2),r0			; do an indirect long move into r0
EXIT:	ret				; return to calling program
	.end

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Neil Rieck
Kitchener - Waterloo - Cambridge, Ontario, Canada.