CPS 104, Ramm - Fall 1999 - 12/3/99 #ASM2

Announce

Assembly Language and the Assembler

Pseudo Ops (assembler instructions)

	DW	Memory allocation and initialize
	END	End of program and Start Address
	EQU	Load Symbol Table
	ORG	Set Loc Ctr
	*	Reference Location Counter

Another Example

	loc	op	address

		ORG	100
	DELTA	EQU	3
	BEGIN	LOAD	X
		ADDI	5
		ADD	Y
		SUBI	DELTA
		ADD	Z
		STORE	ANS
		HALT
		ORG	200
	Z	DW	24
	Y	DW	-4
	ANS	ORG	* + 1
	X	DW	7
		END	BEGIN

Forward Reference Problem
More on symbol table
- other entries
  - (info for loader)
Macros
- Duplicate label problem
  local l1, l2, ...

360 type example

	macro
&l	add3	&r,&t,t&a,&b,&c 	prototype
&l	l	&r,&a
	a	&r,&b
	a	&r,&c
	st	&r,&t
	mend

can be used to change instructions:

	macro
&l	add3w	&r,&t,t&a,&b,&c,&w	w = h, e, d, ''
&l	l&w	&r,&a
	a&w	&r,&b
	a&w	&r,&c
	st&w	&r,&t
	mend

Macros invoking macros

Conditional Assembly (PC Example)

General Form

			
IFxx arg
...
[ELSE]
...
ENDIF

Condition Tested

			
IF expr			; true if expr is not 0
IFE expr		; true if expr is 0
IF1
IF2
IFDEF sym		; true if sym declared external
IFNDEF sym		; true if sym not declared external
IFB 		; true if arg is blank
IFNB 		; true if arg is not blank
IFIDN ,	; true if string arg1 and 2 are ident
IFDIF ,	; true if string arg1 and 2 are dif

Example in Macro

			
add3	macro	t, s1, s2, s3, r
IFDIF ,
	push	r
ENDIF
	mov	r, s1
	add	r, s2
	add	r, s3
	mov	t, r
IFDIF ,
	pop	r
ENDIF
	endm

Linking and Loading

Linking Problem
- External reference
  - Data
Relocation Problem
- where not taken care of by hardware
Position Independent Code
Binding time
- Execution time

Linker Example

			ASSEMBLER OUTPUT	LOADER OUTPUT
ASSEMBLER INPUT		LOADER INPUT
----------------        -------------------     ----------------
Main program
			   MAIN=11R		    *511*
			   DSUB		17
     ORG    11
GO   LOAD   X		11 LOAD 	21R	511 LOAD	521
     STORE  B		12 STORE	22R	512 STORE	522
     ADD    X		13 ADD		21R	513 ADD		521
     STORE  B+1		14 STORE	23R	514 STORE	523
     STORE  B+2		15 STORE	24R	515 STORE	524
     LOAD   Y		16 LOAD		25R	516 LOAD	525
     CALL   DSUB	17 CALL		0	517 CALL	723
     STORE  Y		18 STORE	25R	518 STORE	525
     ...		   ...			    ...
X    DEC    5		21 5			521 5
B    BSS    3			-
Y    DEC    4		25 4			525 4
     END    GO
----------------        -------------------     ----------------
Subroutine
     ENTRY  DSUB	   DSUB=0R
     EXTRN  X,B		 0 X		2,...
     			 1 B		1,...

DSUB STORE  Z		 0 STORE	13R	723 STORE	736
D1   ADD    B+2		 1 ADD	 	2	724 ADD		524
     MPY    X		 2 MPY		0	725 MPY		521
     ...		   ...			    ...
     BRANCH D1		12 BRANCH	1R	735 BRANCH	724
Z    BSS    1			-
     END