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• LOADING: brings the OBJECT PROGRAM into MEMORY for execution 
• Relocation: modifies the object program so that it can be Loaded at an address different from the location originally SPECIFIED 
• Linking: combines two or more separate object programs and also supplies the information needed to reference them. 

The ASSEMBLER directive START GIVES the NAME and STARTING address of the program. 

The format is 

PN START 1000 

Here 

PN — Name of the program 
1000 — Starting address of the program. 

The assembler directive START gives the name and starting address of the program. 

The format is 

PN START 1000 

Here 

PN — Name of the program 
1000 — Starting address of the program. 

CSECT: USED to DIVIDE the PROGRAM into many CONTROL sections 

USE: used to divide the program in to many blocks called program blocks 

CSECT: used to divide the program into many control sections 

USE: used to divide the program in to many blocks called program blocks 

This record gives information about external SYMBOLS that are DEFINED in a particular control section. The FORMAT is 

Col 1 D 

Col 2-7 name of external symbol defined in this control section 

Col 8-13 relative ADDRESS of the symbol with in this control section 

Col 14-73 name and relative address for other external symbols. 

This record gives information about external symbols that are defined in a particular control section. The format is 

Col 1 D 

Col 2-7 name of external symbol defined in this control section 

Col 8-13 relative address of the symbol with in this control section 

Col 14-73 name and relative address for other external symbols. 

EXTDEF names external symbols that are DEFINED in a PARTICULAR control SECTION and may be used in other SECTIONS. EXTREF names external symbols that are referred in a particular control section and defined in another control section. 

EXTDEF names external symbols that are defined in a particular control section and may be used in other sections. EXTREF names external symbols that are referred in a particular control section and defined in another control section. 

A control section is a part of the program that maintains its identity after assembly; each control section can be loaded and RELOCATED INDEPENDENTLY of the others. Control SECTIONS are most often used for subroutines. The major benefit of using control sections is to INCREASE flexibility.

A control section is a part of the program that maintains its identity after assembly; each control section can be loaded and relocated independently of the others. Control sections are most often used for subroutines. The major benefit of using control sections is to increase flexibility.

The ASSEMBLER features that do not depend upon the machine architecture are KNOWN as machine independent assembler features. 

Eg: PROGRAM blocks, Literals. 

The assembler features that do not depend upon the machine architecture are known as machine independent assembler features. 

Eg: program blocks, Literals. 

Assembler PROGRAM can be divided into many sections known as control sections and each control section can be loaded and relocated independently of the others. If the instruction in ONE control section need to refer instruction or data in ANOTHER control section. the assembler is unable to process these REFERENCES in normal way. Such references between control are called EXTERNAL references. 

Assembler program can be divided into many sections known as control sections and each control section can be loaded and relocated independently of the others. If the instruction in one control section need to refer instruction or data in another control section. the assembler is unable to process these references in normal way. Such references between control are called external references. 

This record contains the information about the modification in the object CODE during program relocation, the general format is Col 1 M

Col 2-7 Starting location of the ADDRESS field to be modified relative to the beginning of the program 

Col 8-9 length of the address field to be modified in half bytes. 

This record contains the information about the modification in the object code during program relocation, the general format is Col 1 M

Col 2-7 Starting location of the address field to be modified relative to the beginning of the program 

Col 8-9 length of the address field to be modified in half bytes. 

RESW -It RESERVES the indicated NUMBER of words for data area. 

Eg: 10 1003 THREE RE5W 1 

In this instruction one word area ( bytes) is reserved for the symbol THREE. If die MEMORY is byte addressable then the ADDRESS assigned for die next symbol is 1006. 

RE5B -It reserves die indicated number of bytes for data area. 

Eg: 10 ioo8 INPUT RE5B 1 

In this instruction one byte area is reserved for die symbol INPUT .Hence die address assigned for the next symbol is 1009. 

RESW -It reserves the indicated number of words for data area. 

Eg: 10 1003 THREE RE5W 1 

In this instruction one word area ( bytes) is reserved for the symbol THREE. If die memory is byte addressable then the address assigned for die next symbol is 1006. 

RE5B -It reserves die indicated number of bytes for data area. 

Eg: 10 ioo8 INPUT RE5B 1 

In this instruction one byte area is reserved for die symbol INPUT .Hence die address assigned for the next symbol is 1009. 

A BASE register table is used to remember which of the GENERAL purpose registers are CURRENTLY AVAILABLE as base registers and also the base addresses they contain. 

USING STATEMENT causes entry to the table and .DROP statement removes the corresponding table entry. 

A base register table is used to remember which of the general purpose registers are currently available as base registers and also the base addresses they contain. 

USING statement causes entry to the table and .DROP statement removes the corresponding table entry. 

• Near JUMP: A near jump is a jump to a target in the same SEGMENT and it is assembled by using a current code segment Cs. 
• Far jump: A far jump is a jump to a target in a DIFFERENT code segment and it is assembled by using different segment REGISTERS 

One pass assembler that generates their object code in memory for IMMEDIATE execution is KNOWN as load and GO assembler. Here no object programmer is WRITTEN out and HENCE no need for loader. 

One pass assembler that generates their object code in memory for immediate execution is known as load and go assembler. Here no object programmer is written out and hence no need for loader. 

This VARIABLE is used to assign addresses to the symbols. LOCCTR is initialized to the BEGINNING address specified in the START STATEMENT. After each source statement is processed the length of the assembled instruction or DATA area to be generated is added to LOCCTR and hence whenever we reach a label in the source program the current VALUE of LOCCTR gives the address associated with the label. 

This variable is used to assign addresses to the symbols. LOCCTR is initialized to the beginning address specified in the START statement. After each source statement is processed the length of the assembled instruction or data area to be generated is added to LOCCTR and hence whenever we reach a label in the source program the current value of LOCCTR gives the address associated with the label. 

• CONVERT mnemonic OPERATION codes to their machine LANGUAGE equivalents. 
• Convert symbolic operands to their equivalent machine addresses 
• BUILD the machine instruction in the proper format. 
• Convert the data constants specified in the source program into their internal machine representation 
• WRITE the object program and assembly listing. 

If the result of the expression is an absolute value (CONSTANT) then it is KNOWN as absolute expression. 

Eg: BITFEND - BUFFER 

If the result of the expression is RELATIVE to the beginning of the PROGRAM then it is known as relative expression. label on instructions and data areas and references to the location COUNTER values are relative terms. 

Eg: BUFEND + BUFFER 

If the result of the expression is an absolute value (constant) then it is known as absolute expression. 

Eg: BITFEND - BUFFER 

If the result of the expression is relative to the beginning of the program then it is known as relative expression. label on instructions and data areas and references to the location counter values are relative terms. 

Eg: BUFEND + BUFFER 

An OBJECT PROGRAM that CONTAINS the information necessary to PERFORM REQUIRED modification in the object code depends on the starting location of the program during load time is known as relocatable program. 

An object program that contains the information necessary to perform required modification in the object code depends on the starting location of the program during load time is known as relocatable program. 

• ‘EQU’-it allows the programmer to DEFINE symbols and specify their values directly. The general format is Symbol EQU value 
• ‘ORG’-it is used to indirectly assign values to symbols. When this statement is ENCOUNTERED the assembler resets its location counter to the SPECIFIED value. The general format is ORG value In the above two statements value is a CONSTANT or an expression involving constants and PREVIOUSLY defined symbols. 

The operation code table contains the mnemonic operation code and its machine LANGUAGE equivalent. Some assemblers it may also contain INFORMATION about INSTRUCTION format and LENGTH. OPTAB is usually organized as a hash table, with mnemonic operation code as the key. 

The operation code table contains the mnemonic operation code and its machine language equivalent. Some assemblers it may also contain information about instruction format and length. OPTAB is usually organized as a hash table, with mnemonic operation code as the key. 

The SYMBOL table includes the name and VALUE for each symbol in the source program, together with flags to indicate error CONDITIONS. Sometimes it may contain details about the data area. SYMTAB is usually organized as a hash table for efficiency of insertion and RETRIEVAL.

The symbol table includes the name and value for each symbol in the source program, together with flags to indicate error conditions. Sometimes it may contain details about the data area. SYMTAB is usually organized as a hash table for efficiency of insertion and retrieval.

The header record, text record and the end record are the THREE different records USED in object program. 

1. The header record contains the program name, starting address and length of the program. 
2. Text record contains the translated instructions and data of the program. 
3. End record marks the end of the object program and SPECIFIES the address in the program where execution is to BEGIN. 

The header record, text record and the end record are the three different records used in object program. 

It is a reference to a label that is defined later in a program. 

Consider the statement 

10 1000 STL RETADR 
……
……
8o 1036 RETADR RESW 1 

The first instruction contains a FORWARD reference RETADR. If we ATTEMPT to TRANSLATE the program line by line, we will unable to process the statement in line10 because we do not know the address that will be assigned to RETADR .The address is assigned later(in line 80) in the program. 

It is a reference to a label that is defined later in a program. 

Consider the statement 

10 1000 STL RETADR 
……
……
8o 1036 RETADR RESW 1 

The first instruction contains a forward reference RETADR. If we attempt to translate the program line by line, we will unable to process the statement in line10 because we do not know the address that will be assigned to RETADR .The address is assigned later(in line 80) in the program. 

These are the statements that are not TRANSLATED into MACHINE INSTRUCTIONS, but they provide instructions to assembler itself. 

example S1ART,END.BYTE.WORD.RESW and RESB. 

These are the statements that are not translated into machine instructions, but they provide instructions to assembler itself. 

example S1ART,END.BYTE.WORD.RESW and RESB. 

• Translating mnemonic OPERATION CODES to their machine language equivalents. 
• ASSIGNING machine addresses to symbolic labels used by the PROGRAMMER. 

The test device (TD) instruction tests whether the ADDRESSED device is ready to send or receive a BYTE of data. The condition code is set to indicate the RESULT of this test. Setting of < MEANS the device is ready to send or receive, and = means the device is not ready. 

The test device (TD) instruction tests whether the addressed device is ready to send or receive a byte of data. The condition code is set to indicate the result of this test. Setting of < means the device is ready to send or receive, and = means the device is not ready. 

23 
LDA ALPHA 
ADD SI 
STA BETA 
... ...
ALPHA RESW 1 
BETA RESW 1

23 
LDA ALPHA 
ADD Si 
STA BETA 
... ...
ALPHA RESW 1 
BETA RESW 1

LDA ALPHA 
ADD ONE 
STA BETA
.... ....
ALPHA RESW 1 
BETA RESW 1
ONE RESW 1 

LDA ALPHA 
ADD ONE 
STA BETA
.... ....
ALPHA RESW 1 
BETA RESW 1
ONE RESW 1 

Here the TARGET address is calculated using the FORMULA 

"T.A =(register) + DISPLACEMENT. "

Here the target address is calculated using the formula 

"T.A =(register) + displacement. "

Register direct; register DEFERRED, auto increment and decrement, program counter relative, BASE relative, INDEX register mode and indirect ADDRESSING are the various addressing modes in VAX ARCHITECTURE. 

Register direct; register deferred, auto increment and decrement, program counter relative, base relative, index register mode and indirect addressing are the various addressing modes in VAX architecture. 

The numeric format is used to REPRESENT numeric values with one DIGIT per BYTE. In the numeric format if the sign appears in the last byte it is known as the TRAILING numeric. If the sign appears in a separate byte preceding the first digit then it is called as LEADING separate numeric. 

The numeric format is used to represent numeric values with one digit per byte. In the numeric format if the sign appears in the last byte it is known as the trailing numeric. If the sign appears in a separate byte preceding the first digit then it is called as leading separate numeric. 

In the FIRST instruction IMMEDIATE ADDRESSING is USED. Here the value 3 is directly loaded into the accumulator register. In the second instruction the memory REFERENCE is used. Here the address(address assigned for the symbol THREE) is loaded into the accumulator register. 

In the first instruction immediate addressing is used. Here the value 3 is directly loaded into the accumulator register. In the second instruction the memory reference is used. Here the address(address assigned for the symbol THREE) is loaded into the accumulator register. 

In the line 10 the address is 1000 and the INSTRUCTION is RESW 4. It reserves 4 word (3 X 4 =12) areas for the symbol LENGTH. hence 12 is added to the LOCCTR. Thus the VALUE of the symbol NEW is 1000+ 12 =100C. 

In the line 10 the address is 1000 and the instruction is RESW 4. It reserves 4 word (3 x 4 =12) areas for the symbol LENGTH. hence 12 is added to the LOCCTR. Thus the value of the symbol NEW is 1000+ 12 =100C. 

Format 1 (1 BYTE), Format 2 (2 BYTES), Format 3 (3 bytes) & Format 4(4 bytes) 

Format 2: 

8 4 4 
OPC ODE R1 R2 

Format 1 (1 byte), Format 2 (2 bytes), Format 3 (3 bytes) & Format 4(4 bytes) 

Format 2: 

8 4 4 
OPC ODE R1 R2 

A-accumulator: Used for arithmetic OPERATION. i.e., in the case of arithmetic OPERATIONS one operand is in the accumulator, and other operand may be an immediate VALUE, register operand or memory content. The operation given in the instruction is performed and the result is STORED in the accumulator register. 

L-linkage register: It is used to store the return address in the case of jump to subroutine (JSUB) instructions. 

A-accumulator: Used for arithmetic operation. i.e., in the case of arithmetic operations one operand is in the accumulator, and other operand may be an immediate value, register operand or memory content. The operation given in the instruction is performed and the result is stored in the accumulator register. 

L-linkage register: It is used to store the return address in the case of jump to subroutine (JSUB) instructions. 

SUB 4(direct) to R (direct) 

Here ONE OPERAND is in the ADDRESS LOCATION 4(direct addressing) and the next operand is in the register (register direct). 

The resultant value is 9 -6 =3. 

SUB 4(direct) to R (direct) 

Here one operand is in the address location 4(direct addressing) and the next operand is in the register (register direct). 

The resultant value is 9 -6 =3. 

The loader, which is USED only for loading, is KNOWN as absolute loader. 

e.g. BOOTSTRAP loader 

The loader, which is used only for loading, is known as absolute loader. 

e.g. Bootstrap loader 

CISC -Power PC, CRAY T3E 

RISC -VAX,Pentium Pro architecture 

CISC -Power PC, Cray T3E 

RISC -VAX,Pentium Pro architecture 

In this ADDRESSING mode the operand value is given DIRECTLY. There is no need to refer MEMORY. The immediate addressing is indicated by the PREFIX L#. 

Eg: ADD #5 

In this instruction one operand is in accumulator and the second operand is an immediate value the value is directly added with the accumulator content and the RESULT is stored in accumulator. 

In this addressing mode the operand value is given directly. There is no need to refer memory. The immediate addressing is indicated by the prefix L#. 

Eg: ADD #5 

In this instruction one operand is in accumulator and the second operand is an immediate value the value is directly added with the accumulator content and the result is stored in accumulator. 

In the case of immediate ADDRESSING the operand field gives the memory LOCATION. The word from the given address is fetched and it gives the address of the operand. 

Eg:ADD R5, [600] 

Here the SECOND operand is given in indirect addressing mode. First the word in memory location 600 is fetched and which will give the address of the operand. 

In the case of immediate addressing the operand field gives the memory location. The word from the given address is fetched and it gives the address of the operand. 

Eg:ADD R5, [600] 

Here the second operand is given in indirect addressing mode. First the word in memory location 600 is fetched and which will give the address of the operand. 

OPCODE L B1 D1 B2 D2 
0 7 8 15 16 19 20 31 32 35 36 47 

It is a 6 byte instruction used to MOVE L+I bytes data FRO the storage locationi to the storage location2. 

Storage location1 = D1+[B1] 
Storage location2 = D2+[B2] 

Eg: MOV 60,400(3),500(4) 

opcode L B1 D1 B2 D2 
0 7 8 15 16 19 20 31 32 35 36 47 

It is a 6 byte instruction used to move L+I bytes data fro the storage locationi to the storage location2. 

Storage location1 = D1+[B1] 
Storage location2 = D2+[B2] 

Eg: MOV 60,400(3),500(4) 

MAR (memory address REGISTER) is USED to STORE the address of the memory from which the DATA is to be read or to which the data is to be written.

MAR (memory address register) is used to store the address of the memory from which the data is to be read or to which the data is to be written.

Loader is a set of program that LOADS the MACHINE language TRANSLATED by the TRANSLATOR into the main MEMORY and makes it ready for execution. 

Loader is a set of program that loads the machine language translated by the translator into the main memory and makes it ready for execution. 

COMPILER is a set of program which CONVERTS the whole HIGH level language program to MACHINE language program. Interpreter is a set of programs which converts high level language program to machine language program LINE by line.

Compiler is a set of program which converts the whole high level language program to machine language program. Interpreter is a set of programs which converts high level language program to machine language program line by line.

•  to make the computer easier to use 
•  to manage the resources in computer 
•  process management 
•  DATA and memory management 
•  to provide SECURITY to the user. 

Operating system acts as an interface between the user and the system 

EG :windows,LINUX,unix.dos 

Operating system acts as an interface between the user and the system 

Eg :windows,linux,unix.dos 

It CONSISTS of variety of programs that supports the operation of the computer. This software MAKES it possible for the USER to focus on the other problems to be solved without needing to know how the machine works internally. 

Eg: OPERATING system. assembler, loader. 

It consists of variety of programs that supports the operation of the computer. This software makes it possible for the user to focus on the other problems to be solved without needing to know how the machine works internally. 

Eg: operating system. assembler, loader.