System Programming Unit II

Macro and Macro processor
 Macro:-
 Macro instructions are single line
abbreviations for group of instructions.
 Using a macro, programmer can define
a single “instruction” to represent block
of code.

MACRO -------- Start of definition
INCR -------- Macro name
A 1,DATA
A 2,DATA ------Sequence of instructions to
be
A 3,DATA abbreviated
MEND -------- End of definition

Macro Expansion
 Replacement of macro call by
corresponding sequence of instructions
is called as macro expansion

Source Expanded source
Macro :
Incr :
A 1, data :
A 2, data :
A 3, data :
MEND :
: A 1, Data
: A 2, Data
INCR A 3, Data
: :
: :
INCR :
: A 1, Data
: A 2, Data
data DC F’5’ A 3, Data
: data DC F ‘5’
:
END

Features of macro facility
Macro instructions arguments
:
:
A 1,data1
A 2,data1
A 3,data1
:
:
A 1,data2
A 2,data2
A 3,data2
:
Data1 DC F‟5‟
Data2 DC F‟6‟

Source Expanded Source
MACRO :
INCR &arg (Macro name with :
A 1,&arg one argument) :
A 2,&arg :
A 3,&arg :
MEND A 1,data1
: A 2,data1
: A 3,data1
INCR data1 (use of data1 as :
: operand) A 1,data2
: A 2,data2
INCR data 2 (use of data1 as A 3,data2
: operand) :
data1 DC F’5’ :
data2 DC F’6’ data1 DC F’5’
: data2 DC F’6’
END

Example: more than one
arguments.
A 1,data1
A 2,data2
A 3,data3
:
A 1,data3
A 2,data2
A 3,data1
:
data1 DC F‟5‟
data2 DC F‟6‟

Source Expanded source
MACRO :
& lab1 INCR & arg 1, & arg 2, & arg 3 :
& lab 1 A 1, &arg1 :
A 2, &arg2 :
A 3, &arg3 :
MEND :
: :
: :
: LOOP 1 A 1, data 1
LOOP1 INCR data1, data2, data3 A 2, data 2
: A 3, data 3
: :
: :
LOOP2 INCR data3, data2, data1 LOOP 2 A 1, data3
: A 2, data2
: A 3, data1
data1 DC F ‘5’ :
data2 DC F ‘6’ data1 DC F ‘5’
data3 DC F ‘7’ data2 DC F ‘6’
data3 DC F ‘7’

Two ways of specifying
arguments to a macro call
 A) Positional argument
Argument are matched with dummy
arguments according to order in which
they appear.
 INCR A,B,C
 „A‟ replaces first dummy argument
 „B‟ replaces second dummy argument
 „C‟ replaces third dummy argument

 B) keyword arguments
 This allows reference to dummy
arguments by name as well as by
position.
 e.g.
 INCR &arg1 = A,&arg3 = C, &arg2 =‟B‟
 e.g.
 INCR &arg1 = &arg2 = A, &arg2 =‟C‟

Conditional macro
expansion
 This allows conditional selection of machine
instructions that appear in expansion of macro call.
 The macro processor pseudo op-codes AIF and
AGO help to do conditional macro expansion.
 AIF is conditional branch pseudo op, it performs
arithmetic test and branch only if condition is true.
 AGO is unconditional pseudo op or it is like go to
statement.
 Both statements specify a label appearing in some
other instruction within macro definition.
 These are macro processor directives and they do
not appear in expanded source code

Source Program Expanded Program

:
MACRO
& arg0 INCR & count, & arg1, & arg 2, & arg3
& arg0 A 1, &arg1
AIF (& Count EQ.1). FINAL
A 2,& arg2 LOOP1 A 1, data1
AIF (&count EQ 2). FINAL A 2, data2
A 3, &arg3 A 3, data3
.FINAL MEND :
: :
LOOP1 INCR 3, data1, data2, data3 LOOP2 A 1, data3
: A 2, data2
: :
LOOP2 INCR 2, data3, data2 :
: LOOP3 A 1, data1
LOOP3 INCR 1, data1 :
: data1 DC ‘5’
data1 DC ‘5’ data2 DC ‘6’
data2 DC ‘6’ data3 DC ‘7’
data3 DC ‘7’ :

Macro-calls within macro
 Macros are abbreviations of instruction
sequence. Such abbreviations are also
present within other macro definition.

Source Code Expanded code (level 1) Expanded code (level 2)

:
:
MACRO
ADD 1, &arg
L 1, &arg
A 1, = F ‘10’
ST 1, &arg
MEND
MACRO
ADDS &arg1, &arg2, arg3
ADD1 &arg1
ADD1 &arg2
ADD1 &arg3
MEND Expansion of ADDS
: :
: :
ADDS data1, data2, data3 ADD1 data 1
: ADD1 data 2
: ADD1 data 3
data 1 DC F‘5’ :
END :
data DC F ‘5’
data DC F ‘6’
data DC F ‘7’
END

Source Code Expanded code (level 1) Expanded code (level 2)

:
:
MACRO
ADD 1, &arg
L 1, &arg
A 1, = F ‘10’
ST 1, &arg
MEND
MACRO
ADDS &arg1, &arg2, arg3
ADD1 &arg1 Expansion of ADD1
ADD1 &arg2
ADD1 &arg3 L 1, data 1
MEND Expansion of ADDS A 1, = F „10‟
: : ST 1, data 1
: :
ADDS data1, data2, data3 ADD1 data 1 L 1, data 2
: ADD1 data 2 A 1, = F „10‟
ADD1 data 3 ST 1, data 2
:
L 1, data 3
A 1, = F „10‟
data 3 DC F‘7’ : ST 1, data 3
END :
data DC F ‘5’
data DC F ‘6’ data1 DC F‘5’
data DC F ‘7’ data2 DC F‘6’
END data3 DC F‘7’
END

Macro-instructions defining
macros

Macro-instructions defining
macros
 Macro-definition, which itself is a sequence of instruction,
can be abbreviated by using macro.
 It is important to note that inner macro is not defined until
the outer macro is called.
 The user might call the above macro with the statement-
 DEFINE-MACRO ADD1
 When this call is executed then the macro ADD1 will be
defined. So after the above call user can call the ADD1
macro in following way-
 ADD1 data1
 The macro processor will generate the sequence
L 1, data1
A 1,=F‟0‟
ST 1,data1.

Implementation
 There are four basic task that a macro-processor
must perform
 Recognize macro definition
A macro instruction processor must recognize macro
definition identified by MACRO & MEND pseudo –
operations.
 Save the Definition
The processor must store the macro – definitions which
will be needed at the time of expansion of calls.
 Recognize the Macro call
The processor must recognize macro calls that appear as
operation mnemonics.
 Expand calls & substitute arguments
The macro call is replaced by the macro definition. The
dummy arguments are replaced by the actual data .

Two pass Macro Processor
 General Design Steps
 Step 1: Specification of Problem:-
 Step 2 Specification of databases:-
 Step 3 Specification of database
formats
 Step 4 : Algorithm

Specify the problem
 In Pass-I the macro definitions are
searched and stored in the macro
definition table and the entry is made in
macro name table
 In Pass-II the macro calls are identified
and the arguments are placed in the
appropriate place and the macro calls
are replaced by macro definitions.

Specification of
databases:-
Pass 1:-
 The input macro source program.
 The output macro source program to be used by Pass2.
 Macro-Definition Table (MDT), to store the body of macro
defns.
 Macro-Definition Table Counter (MDTC), to mark next
available entry MDT.
 Macro- Name Table (MNT), used to store names of
macros.
 Macro Name Table counter (MNTC), used to indicate the
next available entry in MNT.
 Argument List Array (ALA), used to substitute index
markers for dummy arguments before storing a macro-
defns.

Specification of
databases:-
 Pass 2:-
 The copy of the input from Pass1.
 The output expanded source to be given to
assembler.
 MDT, created by Pass1.
 MNT, created by Pass1.
 Macro-Definition Table Pointer (MDTP), used
to indicate the next line of text to be used
during macro-expansion.
 Argument List Array (ALA), used to substitute
macro-call arguments for the index markers in
the stored macro-defns

Specification of database
format:-

 MDT is a table of text lines.
 Every line of each macro definition
except the MACRO line, is stored in the
MDT (MACRO line is useless during
macro-expansion)
 MEND is kept to indicate the end of the
depns.
 The macro-name line is retained to
facilitate keyword argument
replacement.

 Each MNT entry consists of

 A character string (the macro name) &

 A pointer (index) to the entry in MDT that
corresponds to the beginning of the macro-
definition.(MDT index)

Argument List Array
(ALA):
 ALA is used during both Pass1 & Pas2 but for some
what reverse functions.
 During Pass1, in order to simplify later argument
replacement during macro expansion, dummy
arguments are replaced with positional indicators
when defn is stored.
Ex. # 1, # 2, # 3 etc.
 The ith dummy argument on the macro-name is
represented in the body by #i.
 These symbols are used in conjunction with ALA
prepared before expansion of a macro-call.
 Symbolic dummy argument are retained on macro-
name to enable the macro processor to handle
argument replacement byname rather by position.

During Pass-II
 During Pass2 it is necessary to substitute
macro call arguments for the index markers
stored in the macro.
 Thus upon encountering the call
expander would prepare a ALA as shown
below:
 LOOP 1NCR DATA1, DATA2, DATA3,}
 → Call in main prog.
 → ALA is prepare of after this call is
encountered.

Algorithm
 Pass1 of macro processor makes a line-by-line scan
over its input.
 Set MDTC = 1 as well as MNTC = 1.
 Read next line from input program.
 If it is a MACRO pseudo-op, the entire macro
definition except this (MACRO) line is stored in MDT.
 The name is entered into Macro Name Table along
with a pointer to the first location of MDT entry of
the definition.
 When the END pseudo-op is encountered all the
macro-defns have been processed, so control is
transferred to pass2

MDTC 1
MNTC 1

Enter Macro Name and
Current value of MDTC in MNT

MDTC MDTC+1

MDTC MDTC+1

Algorithm for Pass – 2
 This algorithm reads one line of i/p prog. at a time.
 for each Line it checks if op-code of that line matches any of the
MNT entry.
 When match is found (i.e. when call is pointer called MDTF to
corresponding macro defns stored in MDT.
 The initial value of MDTP is obtained from MDT index field
of MNT entry.
 The macro expander prepares the ALA consisting of a table of
dummy argument indices & corresponding arguments to the
call.
 Reading proceeds from the MDT, as each successive line is
read, The values form the argument list one substituted for
dummy arguments indices in the macro defn.
 Reading MEND line in MDT terminates expansion of macro &
scanning continues from the input file.
 When END pseudo-op encountered , the expanded source
program is given to the assembler

MACROS PROCEDURE
1
The corresponding machine code 1 The Corresponding m/c code is
is written every time a macro is written only once in memory
called in a program.
2
Program takes up more memory 2 Program takes up comparatively
space. less memory space.
3
No transfer of program counter. 3 Transferring of program counter is
required.
4
No overhead of using stack for 4 Overhead of using stack for
transferring control. transferring control.
5
Execution is fast 5 Execution is comparatively slow.
6
Assembly time is more. 6 Assembly time is comparatively
less.
7
More advantageous to the 7 More advantageous to the
programs when repeated group of programs when repeated group of
instruction is too short. instructions is quite large.

Loaders
 Loader is a program which accepts the
object program decks and prepares
these programs for execution by the
computer and initiates the execution

The loader must perform the
following functions
1. Allocate space in memory for the
program(Allocation)
2. Resolve symbolic references between
object decks(Linking)
3. Adjust all address dependent locations,
such as address constants, to
corresponds to the allocated space
(Relocation)
4. Physically place the machine instruction
and data in memory(Loading)

Loader

A

Loader
A

B B

Data Base

Program Loaded in
memory ready for Execution

Loader Schemes
 Compile-and-Go loader
 General Loader Scheme
 Absolute Loading scheme
 Subroutine Linkages
 Relocating Loaders
 Direct Linking Loaders

Compile-and-Go Loader

Source program
deck

 In this scheme assembler run in one
part of memory and place object
instructions & data, as they are
assembled, directly into their assigned
memory location.
 When assembly is completed,
assembler causes a transfer to the
starting instruction of the program

 Advantages:-
 It is relatively easy to implement.
Assembler simply places code into core and
loader transfer control to starting instruction of
newly assembled program.

 Disadvantages:-
 i) A portion of memory is always occupied by
assembler. That portion is unavailable to any
other object program.
 ii) User‟s program have to be reassemble
every time it is run.
 iii) If source program contains sub routine
written in different languages, then different
assemblers have to be stored in memory ( i.e.
again wastage of memory).

General Loader Scheme
 In this scheme the output of assembler is saved and
loaded whenever code is to be executed.

 Assembled program could be loaded, in the same
area into memory where assembler was stored (
because now assembling is completed assembler is
removed from main memory).

 Function of loader is to accept instruction, data and
other information in object format and put it into
memory in executable format.

 Advantages:-
 No need to put assembler in main memory all the time it can be
removed after assembly is completed. Assembled program is
stored in secondary storage in object form

 Loader is required to put object code in memory in executable
format, which is smaller in size.

 Every time when we run program one do not have to assembler
it every time.

 Even if subroutine are in different languages, their object code &
linkage conventions will be in one language.

 Disadvantage:-
Addition of a new program „Loader‟ into system.

Absolute Loader

100

MAIN 100

248
Absolute MAIN
Loader 248
400
SQRT
478 400
SQRT
Object Deck 478

Main Memory

Absolute Loaders:-
 The simplest type of loader scheme
which fits to the general model is called
as absolute loader.
 Assembler produces object code, which
is stored on secondary storage ( instead
of directly loaded into main memory like
in compile -&-go).
 Loader in turn accepts this object code,
places it into core at prescribed place by
assembler for execution.

Advantages:-
 This scheme makes more memory available to the
user, an assembler is not in memory at load time.
 Simple to implement.
Disadvantages:-
 Programmer must specify to the assembler, the
address of memory location where program is to be
loaded.
 When program contains more than one subroutines,
then programmer must remember the address of
each and have to user it explicitly in other
subroutines to perform subroutine linkages.
 Programmer must be careful not to assign same or
overlapping locations to two subroutines.

Thus in absolute loader scheme
four loader functions are performed
by

 Allocation - by programmer
 Linking - by programmer
 Relocation - by assembler
 Loading - by loader

Subroutine linkage
 The problem is:: A MAIN program A wishes to
transfer to sub-program B. However the
assembler does not know this symbol and
declare it as an undefined symbol (error) unless
a special mechanism has been provided.
 This mechanism has been implemented with a
direct-linking or a relocating loader.
 The assembler pseudo-op EXTRN followed by a
list of symbols indicates that these symbols are
defined in another programs.
 Correspondingly if a symbol is defined in one
program and referenced in others, we insert it
into a symbol list following the pseudo-op
ENTRY.
 The assembler will also inform the loader that
these symbols may be referenced by other
programs.

Subroutine linkage(Cont…1)
 Consider the following example::
MAIN START
EXTRN SUBROUT
----------
L 15, = A(SUBROUT)
BALR 14,15 // reg 14 is return addr of caller.
--------
END
The subroutine can be defined at other location as ::
SUBROUT START
USING *,15
----------
BR 14
END

Relocating loaders
 To avoid possible reassembling of all
subroutines when a single subroutine is
changed, and to perform the task of
allocation and linking for programmer,
relocating loaders are developed.
 In this scheme, assembler assembles
each procedure segment independently
and passes on the text and information
after relocation & inter segment
references to the loader.

 Example:: BSS(Binary Symbolic
Subroutines) loader used in IBM 7094, IBM
1130, etc.
 The o/p of a relocating assembler using a
BSS scheme is the object program &
information about all programs it
references.
 It also provides the relocating information
that needs to be changed if it is placed in
an arbitrary place in core.
 BSS loader scheme is used on computers
with a fixed length direct address
instruction format.

5.RELOCATING LOADER(Cont…1)
SOURCE PROGRAM REL. ADDR RELOCATION OBJECT CODE

MAIN START
EXTRN SQRT 0 00 „SQRT‟
EXTRN ERR 4 00 „ERRb‟
ST 14,SAVE 8 01 ST 14,36
L 1,=F „9‟ 12 01 L 1,40
BAL 14,SQRT 16 01 BAL 14,0
C 1, = F „3‟ 20 01 C 1,44
BNE ERR 24 01 BC 7,4
L 14, SAVE 28 01 L 14,36
BR 14 32 0 BCR 15,14

SAVE DS F 34 0 skipped for alignment
END 36 00 temp. location
40 00
44 00
56

Absolute Relative
Address Address
400 0 BC 15,448
404 4 BC 15, 526
ST 14,436
408 8 L 1,440
412 12 BAL 14,400
C 1,444 LENGTH = 48 BYTES.
416 16
420 20 BC 7,404
L 4,436
424 24 BCR 15,14
428 28 (TEMP LOC)
432 32 9
3
436 36
440 40
444 44 SQRT LENGTH = 78 BYTES.
ERR 57


 Drawbacks ::
1) The transfer vector linkage is only useful
for transfers & is not well situated for
loading or storing external data(data
located in another procedure segments).
2) The transfer vector increases the size of
the object program in memory.

58

6. DIRECT-LINKING LOADER(cont…1)
END
RLD
TXT
ESD
FIG: - OBJECT DESK FOR A DIRECT LINKING LOADER
 ESD – External Symbol dictionary.
 TXT – Text (contains actual assembled program).
 RLD - Relocation & Linkage Directory. – contains
information about those locations in the program
whose contents depends on the address at which
the program is placed. These information are::
 Location of each constant that needs to be
changed due to relocation.
 By what it has to be changed.
 The operation to be performed.
59

DIRECT-LINKING LOADER(cont…2)
 ADVATAGES::
1)It allows the programmer multiple procedure &
data segments.
2)Complete freedom for the programmer to refer the
data in other segments.
3)Provides flexibility in intersegment referencing &
accessing ability.
4)Allows independent translations of the programs.
 DISADVATAGES::
1)It is necessary to allocate, relocate, link & load all
of the subroutines each time in order to execute a
program.
2)These loading are Time-consuming.
3)Although loader is smaller than assembler,
it occupies some memory space.
60

OTHER LOADING SCHEMES
 These includes Binders, linking loaders, Overlays &
Dynamic loaders.
 Binders:: - A Binder is a program that performs the
same functions as that of a direct-linking loader in
binding subroutines together, but rather than placing
the relocated & linked text directly into memory, it
outputs the text as a file or a card deck.
 This o/p file to be loaded is called as “load-module”.
 The functions of binders are ::
 Allocation, Relocation, Linking & Loading.
 There are 2 major classes of Binders::
 Core – image module.( fast & simple)
 Linkage Editor.(complex)
61

Dynamic Loading
overlays
 Each of the previous loader schemes
assume that all of the subroutines are
loaded into core at the same time. It the
total amount of memory available is less
than memory required by all needed
subroutines, then there will be trouble.

Solution to above problem can
be-
 Some hardware techniques like paging
and segmentation are used to solve this
problem
 There is loading scheme called as
dynamic loading which is also available
to solve above problem.

 Usually different subroutines are needed
at different times. E.g. PASS1 & PASS2
of assembler are mutually exclusive.
Both can be loaded one after another,
need not to be loaded at the same time.
 By determining which subroutines call
another, which are exclusive , we can
produce an overlay structure, that will
help to use memory economically.

System Programming Unit II

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System Programming Unit II