System programming note

SYSTEM PROGRAMMING (1ST MODULE)
Prepared By Mr.Santosh Kumar Rath
System programming:It is the activity of programmingsystem software. The
primary distinguishing characteristic of systems programming when compared to application
programming is that application programming aims to produce software which provides services
to the user (e.g. word processor), whereas systems programming aims to produce software which
provides services to the computer hardware (e.g. disk defragmenter). It requires a greater degree
of hardware awareness.
Types of software programs
System software helps use the operating system and computer system. It includes diagnostic
tools, compilers, servers, windowing systems, utilities, language translator, data communication
programs, database systems and more. The purpose of system software is to insulate the
applications programmer as much as possible from the complexity and specific details of the
particular computer being used, especially memory and other hardware features, and such
accessory devices as communications, printers, readers, displays, keyboards, etc.
1.Application Software:Application software, also known as an application or an "app", is
computer software designed to help the user to perform singular or multiple related specific
tasks. It helps to solve problems in the real world. Examples include enterprise software,
accounting software, office suites, graphics software, and media players.
Application software is contrasted with system software and middleware, which manage and
integrate a computer's capabilities, but typically do not directly apply them in the performance of
tasks that benefit the user. A simple, if imperfect, analogy in the world of hardware would be the
relationship of an electric light bulb (an application) to an electric power generation plant (a
system). The power plant merely generates electricity, not itself of any real use until harnessed to
an application like the electric light that performs a service that benefits the user.
 Spreadsheets: The spreadsheet packages are designed to use numbers and formulas to do
calculations with ease. Examples of spreadsheets include:
 Budgets
 Payrolls
 Grade Calculations
 Address Lists
The most commonly used spreadsheet programs are Microsoft Excel and Lotus 123.
Graphic Presentations: The presentation programs can be easier using overhead
projectors. Other uses include:
 Slide Shows
 Repeating Computer Presentations on a computer monitor
 Using Sound and animation in slide shows

The most recognized graphic presentation programs are Microsoft PowerPoint and Hardware
Graphics.
 Database Management System (DBMS):
 A DBMS is a software tool that allows multiple users to store, access, and process data
into useful information.
 Database programs are designed for these types of applications:
 Membership lists
 Student lists
 Grade reports
 Instructor schedules
All of these have to be maintained so you can find what you need quickly and accurately.
Example: Microsoft Access, dBase, Oracle.
2. System software: System software is computer software designed to operate the computer
hardware and to provide maintain a platform for running
• The computer
application software.
The most basic types of system software are:
BIOS and device firmware, which provide basic functionality to operate
and control the hardware connected to or built into the computer.
• The operating system (prominent examples being Microsoft Windows, Mac OS X and
Linux), which allows the parts of a computer to work together by performing tasks like
transferring data between memory and disks or rendering output onto a display device. It
also provides a platform to run high-level system software and application software.
• Utility software, which helps to analyze, configure, optimize and maintain the computer.
In contrast to system software, software that allows users to do things like create text documents,
play games, listen to music, or surf the web is called application software.
Specific kinds of system software include:
• Loaders
• Linkers
• Utility software
• Desktop environment / Graphical user interface
• Shells
• BIOS
• Hypervisors
• Boot loaders
• Database Management Systems(SQL, MySQL)
If system software is stored on non-volatile memory such as integrated circuits, it is usually
termed firmware.

Evolution of components of a programming system
Assembler: -- An assembler is a program that takes basic computer instructions and converts
them into a pattern of bits that the computer's processor can use to perform its basic operations.
Some people call these instructions assembler language and others use the term assembly
language.
Here's how it works:
• Most computers come with a specified set of very basic instructions that correspond to
the basic machine operations that the computer can perform. For example, a "Load"
instruction causes the processor to move a string of bits from a location in the processor's
memory to a special holding place called a register. Assuming the processor has at least
eight registers, each numbered, the following instruction would move the value (string of
bits of a certain length) at memory location 3000 into the holding place called register 8:
L 8, 3000
• The programmer can write a program using a sequence of these assembler instructions.
• This sequence of assembler instructions, known as the source code or source program, is
then specified to the assembler program when that program is started.
• The assembler program takes each program statement in the source program and
generates a corresponding bit stream or pattern (a series of 0's and 1's of a given length).
• The output of the assembler program is called the object code or object program relative
to the input source program. The sequence of 0's and 1's that constitute the object
program is sometimes called machine code.
• The object program can then be run (or executed) whenever desired.
 A utility program called an assembler is used to translate assembly language statements into
the target computer's machine code. The assembler performs a more or less isomorphic
People
Application Programming
Compiler Assemblers Macro Processors
Loaders Text Editors Searching and SortingDebugging aids
I/O programs Memory
Management
LibrariesSchedulerFile Systems Device Management

translation (a one-to-one mapping) from mnemonic statements into machine instructions and
data. This is in contrast with high-level languages, in which a single statement generally
results in many machine instructions.
 An assembly languageis a low-level programming language for computers, microprocessors,
microcontrollers, and other integrated circuits. It implements a symbolic representation of the
binary machine codes and other constants needed to program a given CPU architecture. This
representation is usually defined by the hardware manufacturer, and is based on mnemonics
that symbolize processing steps (instructions), processor registers, memory locations, and
other language features. An assembly language is thus specific to a certain physical (or
virtual) computer architecture. This is in contrast to most high-level programming languages,
which, ideally, are portable.
Loader:
In a computer operating system , a loader is a component that locates a given program (which
can be an application or, in some cases, part of the operating system itself) in offline storage
(such as a hard disk ), loads it into main storage (in a personal computer, it's called random
access memory ), and gives that program control of the computer (allows it to execute its
instruction s).
A program that is loaded may itself contain components that are not initially loaded into main
storage, but can be loaded if and when their logic is needed. In a multitasking operating system, a
program that is sometimes called a dispatcher juggles the computer processor's time among
different tasks and calls the loader when a program associated with a task is not already in main
storage. (By program here, we mean a binary file that is the result of a programming language
compilation, linkage editing, or some other program preparation process.)
In computing, a loader is the part of an operating system that is responsible for loading programs,
one of the essential stages in the process of starting a program, it means loader is a program that
places programs into memory and prepares them for execution. Loading a program involves
reading the contents of executable file, the file containing the program text, into memory, and
then carrying out other required preparatory tasks to prepare the executable for running. Once
loading is complete, the operating system starts the program by passing control to the loaded
program code.
All operating systems that support program loading have loaders, apart from systems where code
executes directly from ROM or in the case of highly specialized computer systems that only have
a fixed set of specialized programs.
In many operating systems the loader is permanently resident in memories, although some
operating systems that support virtual memory may allow the loader to be located in a region of
memory that is pageable.

In the case of operating systems that support virtual memory, the loader may not actually copy
the contents of executable files into memory, but rather may simply declare to the virtual
memory subsystem that there is a mapping between a region of memory allocated to contain the
running program's code and the contents of the associated executable file. (See memory-mapped
file.) The virtual memory subsystem is then made aware that pages with that region of memory
need to be filled on demand if and when program execution actually hits those areas of unfilled
memory. This may mean parts of a program's code are not actually copied into memory until
they are actually used, and unused code may never be loaded into memory at all.
Macro:
In computer science is a rule or pattern that specifies how a certain input sequence (often a
sequence of characters) should be mapped to an output sequence (also often a sequence of
characters) according to a defined procedure. The mapping processes that instantiates
(transforms) a macro into a specific output sequence is known as macro expansion.
The term originated with macro-assemblers, where the idea is to make available to the
programmer a sequence of computing instructions as a single program statement, making the
programming task less tedious and less error-prone. Macros often allow positional or keyword
parameters that dictate what the conditional assembler program generates and have been used to
create entire programs or program suites according to such variables as operating system,
platform or other factors.
Compiler:
A compiler is a special program that processes statements written in a particular programming
language and turns them into machine language or "code" that a computer's processor uses.
Typically, a programmer writes language statements in a language such as Pascal or C one line at
a time using an editor. The file that is created contains what are called the source statements. The
programmer then runs the appropriate language compiler, specifying the name of the file that
contains the source statements.
When executing (running), the compiler first parses (or analyzes) all of the language statements
syntactically one after the other and then, in one or more successive stages or "passes", builds the
output code, making sure that statements that refer to other statements are referred to correctly in
the final code. Traditionally, the output of the compilation has been called object code or
sometimes an object module. (Note that the term "object" here is not related to object-oriented
programming .) The object code is machine code that the processor can process or "execute" one
instruction at a time.
A program that translates source code into object code. The compiler derives its name from the
way it works, looking at the entire piece of source code and collecting and reorganizing the
instructions. Thus, a compiler differs from an interpreter, which analyzes and executes each line
of source code in succession, without looking at the entire program. The advantage of
interpreters is that they can execute a program immediately. Compilers require some time before

an executable program emerges. However, programs produced by compilers run much faster
than the same programs executed by an interpreter.
Every high-level programming language (except strictly interpretive languages) comes with a
compiler. In effect, the compiler is the language, because it defines which instructions are
acceptable.
Because compilers translate source code into object code, which is unique for each type of
computer, many compilers are available for the same language. For example, there is a
FORTRAN compiler for PCs and another for Apple Macintosh computers. In addition, the
compiler industry is quite competitive, so there are actually many compilers for each language
on each type of computer. More than a dozen companies develop and sell C compilers for the
PC.
Linker: -- Linker performs the linking of libraries with the object code to make the object code
into an executable machine code.
Formal system:
In formal logic, a formal system (also called a logical calculus[citation needed]) consists of a
formal language and a set of inference rules, used to derive (to conclude) an expression from one
or more other premises that are antecedently supposed (axioms) or derived (theorems). The
axioms and rules may be called a deductive apparatus. A formal system may be formulated and
studied for its intrinsic properties, or it may be intended as a description (i.e. a model) of external
phenomena. A compiler is a computer program (or set of programs) that transforms source code
written in a programming language (the source language) into another computer language (the
target language, often having a binary form known as object code). The most common reason for
wanting to transform source code is to create an executable program.
Each formal system has a formal language, which is composed by primitive symbols. These
symbols act on certain rules of formation and are developed by inference from a set of axioms.
The system thus consists of any number of formulas built up through finite combinations of the
primitive symbols—combinations that are formed from the axioms in accordance with the stated
rules.
Formal systems in mathematics consist of the following elements:
1. A finite set of symbols (i.e. the alphabet), that can be used for constructing formulas (i.e.
finite strings of symbols).
2. A grammar, which tells how well-formed formulas (abbreviated wff) are constructed out
of the symbols in the alphabet. It is usually required that there be a decision procedure for
deciding whether a formula is well formed or not.
3. A set of axioms or axiom schemata: each axiom must be a wff.
4. A set of inference rules.

A formal system is said to be recursive (i.e. effective) if the set of axioms and the set of inference
rules are decidable sets or semidecidable sets, according to context.
Some theorists use the term formalism as a rough synonym for formal system, but the term is
also used to refer to a particular style of notation, for example, Paul Dirac's bra-ket notation
Operating Systems
 Operating System is software, which makes a computer to actually work.
What is OS?
 It is the software the enables all the programs we use.
 The OS organizes and controls the hardware. Examples:
An operating system (OS) is
Windows, Linux, Unix and Mac
OS, etc.,
software, consisting of programs and data, that runs on computers
and manages computer hardware resources[
and provides common services for efficient execution
of various application software.For hardware functions such as input and output and memory
allocation, the operating system acts as an intermediary between application programs and the
computer hardware,although the application code is usually executed directly by the hardware,
but will frequently call the OS or be interrupted by it. Operating systems are found on almost any
device that contains a computer—from cellular phones and video game consoles to
supercomputers and web servers. Examples of popular modern operating systems for personal
computers are Microsoft Windows, Mac OS X, and GNU/Linux.
 controlling and allocating memory,
What OS does?
An operating system performs basic tasks such as,
 prioritizing system requests,
 controlling input and output devices,
 facilitating networking and
 Managing file systems.

The structure of OS consists of 4 layers:
Hardware: Hardware consists of CPU, Main memory, I/O Devices, etc,
Software (Operating System): Software includes process management routines, memory
management routines, I/O control routines, file management routines.
System programs: This layer consists of compilers, Assemblers, linker etc.
Application programs: This is dependent on users need. Ex. Railway reservation system,
Bank database management etc,
Evolution of OS
Structure of Operating System:
 The evolution of operating systems went through seven major phases.
 Six of them significantly changed the ways in which users accessed computers through the
open shop, batch processing, multiprogramming, timesharing, personal computing, and
distributed systems.
 In the seventh phase the foundations of concurrent programming were developed and

Timesharing Simultaneoususer interaction, On-
line file systems
Mastics file system (1965),Unix
(1974)
Concurrent
Programming
Hierarchical systems, Extensible
kernels, Parallel programming
concepts, Secure parallel languages
RC 4000 system (1969),13 Venus
system (1972),14 Boss 2 system
(1975).
Personal
Computing
Graphic user interfaces OS 6 (1972),Pilot system (1980)
Distributed
Systems
Remote servers WFS file server (1979) Unix
United RPC (1982)24 system .
 In Batch processing same type of jobs batch (BATCH- a set of jobs with similar needs)
together and execute at a time.
Batch Processing:
 The OS was simple, its major task was to transfer control from one job to the next.
 The job was submitted to the computer operator in form of punch cards. At some later time
the output appeared.
 The OS was always resident in memory. (Ref. Fig. next slide)
 Common output devices were line printers, tape drives, and card punches.
 Users did not interact directly with the computer systems, but he prepared a job (comprising
of the program, the data, & some control information).
 Multiprogramming is a technique to execute number of programs simultaneously by a
single processor.
Multiprogramming:
 In Multiprogramming, number of processes resides in main memory at a time.
 The OS picks and begins to execute one of the jobs in the main memory.
Major
Phases
Technical Innovations Operating Systems
Open Shop The idea of OS IBM 701 open shop (1954)
Batch
Processing
Tape batching, First-in, first-out
scheduling.
BKS system (1961)
Multi-
programming
Processor multiplexing, Indivisible
operations, Demand paging,
Input/output spooling, Priority
scheduling, Remote job entry
Atlas supervisor (1961),
Exec II system (1966)

 If any I/O wait happened in a process, then CPU switches from that job to another job.
 Hence CPU in not idle at any time.
 The main memory consists of 5 jobs at a time, the CPU executes one by one.
• Efficient memory utilization
Advantages:
• Throughput increases,
• CPU is never idle, so performance increases
 Time sharing, or multitasking, is a logical extension of multiprogramming.
Time Sharing Systems:
 Multiple jobs are executed by switching the CPU between them.
 In this, the CPU time is shared by different processes, so it is called as “Time sharing
Systems”.
 Time slice is defined by the OS, for sharing CPU time between processes.
 Examples: Multics, Unix, etc.,
Operating Systems functions:
1. Program creation
The main functions of operating systems are:
2. Program execution
3. Input/output operations
4. Error detection
5. Resource allocation
6. Accounting
7. Protection
 Single User Systems
Types of OS:
Operating System can also be classified as,-
 Multi User Systems
 Provides a platform for only one user at a time.
Single User Systems:
 They are popularly associated with Desk Top operating system which run on standalone
systems where no user accounts are required.
 Example: DOS

 Provides regulated access for a number of users by maintaining a database of known
users.
Multi-User Systems:
 Refers to computer systems that support two or more simultaneous users.
 Another term for multi-user is time sharing.
 Ex: All mainframes and are multi-user systems.
Example: UNIX
UNIX Operating Systems
• It is a modified KDE desktop operating under Linux.
• The Unix-like family is a miscellaneous group of Unix Operating Systems, with several
major sb-categories including BSD, System V, and Linux.
• UNIX systems run on a wide variety of machine structural designs.
• They are utilized heavily as server systems in business, as well as workstations in
educational and engineering environments.
• Complementary software UNIX options are Linux and BSD, are famous in these areas.
Microsoft Windows Operating Systems
• The Microsoft Windows family of Operating Systems derived as append to the older
MS-DOS milieu for the IBM PC.
• Contemporary versions are based on the newer Windows NT core that was initially
intended for OS/2 and borrowed from VMS. Windows runs on x86, x86-64 and Itanium
processors.
• Previous versions also operate on the MIPS, DEC Alpha, Fairchild Clipper and PowerPC
structural designs.
Sun Solar Operating Systems
• One of the world's largest on-hand inventories of fully tested, renovated Sun
Microsystems paraphernalia.
• Solar Systems Peripherals, Inc. is devoted to providing user with outstanding values in
Sun Microsystems.

General Machine Structure
The structure above consists of -
1. Instruction Interpreter
2. Location Counter
3. Instruction Register
4. Working Registers
5. General Register
The Instruction Interpreter Hardware is basically a group of circuits that perform the operation
specified by the instructions fetched from the memory. The Location Counter can also be called as
Program/Instruction Counter simply points to the current instruction being executed.
The working registers are often called as the "scratch pads" because they are used to store temporary
values while calculation is in progress. This CPU interfaces with Memory through MAR & MBR
MAR (Memory Address Register) - contains address of memory location (to be read from or stored into)
MBR (Memory Buffer Register) - contains copy of address specified by MAR Memory controller is used
to transfer data between MBR & the memory location specified by MAR. The role of I/O Channels is to
input or output information from memory.

Memory Register
• Program Count Register (PC):Also called instruction pointer
• Instruction Register (IR) :Stores instruction fetched from memory
• Memory Address Register (MAR)
• Memory Data Register (MDR)
• Status Registers
• Status of CPU and currently executing program
• Flags (one bit Boolean variable) to track condition like arithmetic carry and overflow, power
failure, internal computer error
A. Register Operations
• Stores values from other locations (registers and memory)
• Addition and subtraction
• Shift or rotate data
• 󲐀Test contents for conditions such as zero or positive
B. Operation of Memory
• Each memory location has a unique address
• Address from an instruction is copied to the
• MAR which finds the location in memory
• CPU determines if it is a store or retrieval
• Transfer takes place between the MDR and memory
• MDR is a two way register
Data and instructions
Instructions and data
Our CPU processes instructions and data. It receives orders from the software. The CPU is fed a
gentle stream of binary data via the RAM.
These instructions can also be called program code. They include the commands which you
constantly – via user programs – send to your PC using your keyboard and mouse. Commands to
print, save, open, etc.
Data is typically user data. Think about that email you are writing. The actual contents (the text,
the letters) are user data. But when you and your software say “send”, your are sending program
code (instructions) to the processor.

Each instruction of a computer specifies an operation on certain data. There are various ways of
specifying address of the data to be operated on. An addressing mode specifies how to calculate
the effective memory address of an operand by using information held in
Address Generated By IBM system 370:
In System/370, a 24-bit virtual address is generated. The address is the address of a byte but may
actually refer to one or more bytes depending on the op code.
What is a Addressing Mode?
registers and/or
constants contained within a machine instruction or elsewhere.
The various addressing modes that are defined in a given instruction set architecture define how
machine language instructions in that architecture identify the operand (or operands) of each
instruction.
These different ways of specifying data are called the addressing modes. The most common
addressing modes are:
• Immediate addressing mode
• Direct addressing mode
• Indirect addressing mode
• Register addressing mode
• Register indirect addressing mode
• Displacement addressing mode
To specify the addressing mode of an instruction several methods are used. Most often used are:
a) Different operands will use different addressing modes.
b) One or more bits in the instruction format can be used as mode field. The value of the mode
field determines which addressing mode is to be used.
The effective address will be either main memory address of a register.
Immediate Addressing:
This is the simplest form of addressing. Here, the operand is given in the instruction itself. This
mode is used to define a constant or set initial values of variables. The advantage of this mode is
that no memory reference other than instruction fetch is required to obtain operand. The
disadvantage is that the size of the number is limited to the size of the address field, which most
instruction sets is small compared to word length.

INSTRUCTION
OPERAND
Direct Addressing:
In direct addressing mode, effective address of the operand is given in the address field of the
instruction. It requires one memory reference to read the operand from the given location and
provides only a limited address space. Length of the address field is usually less than the word
length.
Ex : Move P, Ro, Add Q, Ro P and Q are the address of operand.
Indirect Addressing:
Indirect addressing mode, the address field of the instruction refers to the address of a word in
memory, which in turn contains the full length address of the operand. The advantage of this
mode is that for the word length of N, an address space of 2N can be addressed. He disadvantage
is that instruction execution requires two memory reference to fetch the operand Multilevel or
cascaded indirect addressing can also be used.
Register Addressing:
Register addressing mode is similar to direct addressing. The only difference is that the address
field of the instruction refers to a register rather than a memory location 3 or 4 bits are used as
address field to reference 8 to 16 generate purpose registers. The advantages of register
addressing are Small address field is needed in the instruction.
Register Indirect Addressing:
This mode is similar to indirect addressing. The address field of the instruction refers to a
register. The register contains the effective address of the operand. This mode uses one memory
reference to obtain the operand. The address space is limited to the width of the registers
available to store the effective address.
Displacement Addressing:
In displacement addressing mode there are 3 types of addressing mode. They are :
1) Relative addressing
2) Base register addressing
3) Indexing addressing.
This is a combination of direct addressing and register indirect addressing. The value contained
in one address field. A is used directly and the other address refers to a register whose contents
are added to A to produce the effective address.

Addressing Modes Supported by IBM 360:
Three basic addressing modes are supported: immediate, register, and storage.
 An immediate operand is a 1-byte value contained in some instructions.
Types of operands involved:
 A register operand is contained in a general register or a floating-point register, depending on
the instruction.
 A storage operand is contained in virtual memory. Each storage-operand reference consists of a
12-bit displacement and a 4-bit register identifier that designates one of the general registers. If
the register identifier is 0, it indicates that the register is component is not to be used.
 General: fixed-point arithmetic instruction, logical instruction, general register shifting
instruction, general register load & store instructions, compare instruction, branch instructions,
conversion instructions.
Instruction Types Supported :
The System/370 principles of operations define five broad classes of machine instructions:
 Decimal: decimal instructions
 Floating point: floating-point instructions
 Privileged I/O: special-purpose control instructions, privileged I/O instructions
 Privileged System Control: privileged system-control instructions
Base Registers in address calculation:
1. A 32–bit binary arithmetic, as described by IBM:
The System/370 uses 32–bit two’s–complement signed integers. The range of these integers is –
231
to
231
– 1, inclusive. Now, 231
= 2,147,483,648, so the range is –2,147,483,648 through 2,
147,483,6647.
Each of the sixteen general–purpose registers contains binary integer data. The bits in the
registers are numbered left to right. But 0 is the sign bit. It is 1 for a negative number and 0 for a
non–negative.

2. A characterization of the sixteen general–purpose registers in the
System/370. Which are really general purposes?
Addressing:
The general purpose registers perform all the addressing involved in referencing main storage.
When used for addressing, the register is called a base register.
When so used, the contents of the register are viewed as an unsigned integer. Some
System/370 systems provide only 24 bits for addressing.Current System/390 systems use 64–bit
addressing.
The General Purpose Registers(Some facts):
The general–purpose registers in the System/370 are identified by number: 0 – 15.Of these, only
the ten registers 3 through 12 can be used for any purpose. The other six registers are “less
general purpose” and should be used with caution. Registers 0 and 1 can be used as temporary
registers, but calls to supervisor routines will destroy their contents. Register 2 can be used as a
temporary and possibly as a base register.
The TRT (Translate and Test) instruction will change the value of this register. Registers 13, 14,
and 15 are used by the control programs and subprograms. Each of the sixteen registers is
identified by a four–bit binary number, or equivalently by a single hexadecimal digit. Suggested
convention: Use either register 3 or register 12 as the single required base register.
The standard prefix code should contain the following sequence.
BALR 12, 0 This uses register 12 as a base.
USING *, 12
Register Naming Conventions
Note that there are two ways of writing the part of the standard prefix code.
What we have discussed in these slides:
BALR 12, 0
USING *, 12
What we have used in our labs is valid if it follows a “R12 EQU 12”:
BALR R12, 0
USING *, R12
From this I infer that the assembler will accept either notation in places
in which the syntax expects a register to be used.
NOTE: The first line is a truncated subroutine call.
The instruction says: 1. Store the address of the next instruction into R12
2. Branch to the address in the second argument.
Since that second address is 0, the branch is not taken and execution continues.

1. A base address, stored in a specified base register.
In general, only registers 3 through 12 should be used as base registers.
Base Register Addressing:
The System/370 uses a common design feature that splits addresses into two parts:
2. A displacement, specifying the positive offset (in bytes) from the start
of the section. The System/370 uses a 12–bit number for this displacement.
The displacement value is in the range 0 through 4095, inclusive.
The format of the address in this form is as follows:
| B | D D D |
where B is the single hexadecimal digit indicating the base register, and “D D D” denotes the
three hexadecimal digits used to specify the offset. Suppose that general-purpose register 3
contains the value X’4500’.The address reference 3507, shown as | 3 | 507 |
refers to the address X’4500’ + X’507’ = X’4A07’ In hexadecimal 5 + 5 = A.
NOTE: Register 0 cannot be used as a base register. The assembler will interpret
the address | 0 | D D D | as no base register being used.
BALR R0, 0
USING *, R0
Any call to a system procedure will disrupt the addressing.
Options: No Base Register vs. The Default Base Register
So far, we have considered only the object code form of a typical address. We now “jump
ahead” a bit and look at two typical instructions that use this address type.One type of
instruction, called “RS”, used for register–to–storage instructions.Such an instruction has source
code of the form OP R1,R3,D2(B2).Such an instruction has object code of the form OP R1R3
B2D2 D2D2.
We look at LM, an interesting example of this format.
LM R1,R3,S2 loads multiple registers in the range R1 – R3 from the memory
location specified by S2, the address of which will be in the form | B2 | D2 D2 D2 |.
We now interpret the following code fragment.
BALR R12, 0 Establish register R12 (X‘C’)
USING *, R12 as the default base resister.
LM R5,R7,S2 might have object code 98 57 C1 00.
This uses the default base register. LM R9,R11,S3(R3) might have object code 98 9B 32 00.
Object code such as 98 9B 0E 00 would call for use of an absolute address, not computed from a
base register. For this example, it is likely to be bad code.

Advantage Of Using Base Register Addressing:
It offers two advantages of base/displacement addressing.
Remember that the System/370 of the time admitted a 24–bit address space, with
addresses ranging from 0 through 224
–1 or 0 through 16,777,215.
A full 24–bit address would require 24 bits, or six hexadecimal digits, or three bytes.
The base register/displacement method of addressing allocates
4 bits to the base register
12 bits to the displacement
In this method, an address requires 16 bits, or two bytes.
We now quote Abel on the first major advantage of base/displacement addressing “The
instruction length is reduced because each address requires only two bytes rather than three.”
One might infer that some of the System/360 and System/370 installations had very little
memory. Base Register/Displacement Addressing: Relocating the Code The second major
advantage of base/displacement addressing still applies today. “The system facilitates program
relocatability. Instead of assigning specific [fixed] storage addresses, the assembler
determines each address relative to a base address. At execute time [after the program is loaded],
the base address, which may be anywhere in storage, is loaded into a base register.”
The standard prefix code
BALR 3, 0
USING *, 3
Not an instruction; no address assigned may be translated as follows:
Q. What is the address of the first instruction after the “USING”.
Load that address into register 3 and use it as a base address in that register.
Base/Displacement vs. Indexed Addressing:
Note the similarities with indexed addressing, in which the base is given by a variable and the
offset is given by a register. Systems that use the register contents as a base do so because the
registers can store larger numbers than the bits in the machine code. For example, the
System/360 allocates only 12 bits for the displacement, which is combined with the contents of a
register, which can be a 32–bit number.
Addressing Example:
Here are some more examples of addressing using an index register and a base register. All of
these examples are taken from type RX instructions, which use indexing. length of the form OP
R1,D2(X2,B2-------4byte. The format of the object code is OP R1X2 B2D2 D2D2. Each byte
contains two hexadecimal digits.
We interpret the 32–bit object code as follows.
OP ------8 bit operation code.

R1X2 ---------R1 denotes a register as the source or destination of the operation.
X2 denotes a general–purpose register to be used as an index register.
B2 D2 D2D2 This contains the argument address as a base register and displacement.
Remember that the displacement, given by three hexadecimal digits, is treated as a
12–bit unsigned integer. In decimal, the limit is 0 ≤ Displacement ≤ 4095.
The general form by which an address is computed is
Contents (Base Register) + Contents (Index Register) + Displacement.
Some instructions do not use index register addressing.
Addressing: Example 1:
Here is some object code for analysis.
58 40 C1 23
The first thing to note is that the opcode, 58, is that for L, a Register Load. This is a type RX
instruction with object code of the form OP R1X2 B2D2 D2D2.
As noted above, OP = 58.
We see that R1 = 4. It is register 4 that is being loaded from memory. We see that X2 = 0.
Indexed addressing is not used. We also note that B2D2 D2D2 = C1 23, indicating an offset of
X‘123’ from the address value stored in general–purpose register 12 (hexadecimal C).
Suppose that the value in general–purpose register 12 is X‘2500’. The effective address for this
instruction is then X‘2500’ + X‘123’ = X‘2623’.
Addressing: Example 2:
Here is another example of object code.
58 A7 B1 25
The first thing to note is that the opcode, 58, is that for L, a Register Load. This is a type RX
instruction with object code of the form OP R1X2 B2D2 D2D2.
As noted above, OP = 58.
The hexadecimal digit for the register is A, indicating that register 10 is being loaded. Recall that
all of the digits in the object code are given in hexadecimal. We see that X2 = 7, indicating that
general–purpose register 7 is being used as an index register. We also note that B2D2 D2D2 = B1
25, indicating an offset of X‘125’ from the address value stored in general–purpose register 11
(hexadecimal B).
Suppose the following: Register 11 contains X‘0012 4000’
The displacement is X‘0000 0125’
Register 7 contains X‘0000 0300’
The address is thus X‘0012 4425’

Selecting and Assigning Address to a base register:
Again, the standard prefix code handles this.
BALR 12, 0
USING *, 12
If register 12 is used as a base register, it cannot be used for any other purpose.In other words,
your code should not reference register 12 explicitly.We have two standards suggested for a base
register
SAVE (14,12) SAVE CALLER'S REGS
BALR R12,0 ESTABLISH
USING *,R12 ADDRESSABILITY
LA R2,SAVEAREA POINT TO MY LOWER-LEVEL SA
ST R2,8(,R13) FORWARD-CHAIN MINE FROM CALLER'S
ST R13,SAVEAREA+4 BACK-CHAIN CALLER'S FROM MINE
LR R13,R2 SET 13 FOR MY SUBROUTINE CALLS
********************** BEGIN LOGIC ***********************
NOTE that this code uses register 12 as the base register.
Introduction to Assembly Language Programming
Assembly programming is low-level programming using some basic syntax to represent machine
code for a specific CPU. An assembler is used to translate the assembly code into the machine
code for the target computer.
A program created from assembly can be more efficient and faster than a program created with a
compiler. One example of why this occurs is because a compiler will store intermediate values
used in a calculation in memory whereas a program written in assembly can store the
intermediate values in a register which is much faster. However, while there may be performance
benefits to programming so close to the machine level, there is a great deal of added complexity
which a high-level programming language can remove.

System programming note

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