1. Assembly Language
Prakash Khaire
UTU

2. Assembly Language
 A programming language that is one step away from machine
language.
 Each assembly language statement is translated into one
machine instruction by the assembler.
 Programmers must be well versed in the computer's
architecture, and, undocumented assembly language
programs are difficult to maintain.
 It is hardware dependent; there is a different assembly
language for each CPU series.

3. Assembly Language
 It was introduced in 1952
 Helped to overcome machine language
programming
Definition : “A language that allows
instructions and storage locations to be
represented by letters and symbols instead of
numbers is called assembly language or
symbolic language”

4. Assembly Language Model
…
add r1,r2
sub r2,r3
PC cmp r3,r4
ALU
bne I1 Registers Memory
sub r4,1
I1: jmp I3
…

5. Assembly Language Instructions
Built from two pieces
Add R1, R3, 3
Opcode Operands
What to do with Where to get
the data data and put
(ALU operation) the results

6. Types of Opcodes
Arithmetic, logical
add, sub, mult
and, or
Cmp
Memory load/store
ld, st
Control transfer
jmp
bne

7. Operands
Each operand taken from a particular
addressing mode:
Examples:
Register add r1, r2, r3
Immediate add r1, r2, 10
Indirect mov r1, (r2)
Offset mov r1, 10(r3)
PC Relative beq 100
Reflect processor data pathways

8. Advantages over Machine Language
Easier to understand and use
Easier to locate and correct errors.
Easier to modify
No worry about addresses
Easily relocatable
Efficiency of machine language

9. Limitations of Assembly Language
Machine Dependent
Knowledge of hardware required
Machine level coding

10. Architecture of 8086

11. Registers in 8086 CPU
 There are four general purpose registers
 AX [divided into AH/AL] – the accumulator register
 BX [divided into BH/BL] – the base address register
 CX [divided into CH/CL] – the count register
 DX [divided into DH/DL] – the data register
Other registers
 SI – source index register
 DI – destination index register
 BP – Base pointer
 SP – Stack pointer

12. Registers in 8086 CPU
 This registers are of 16 bits and divided into two 8 bit registers
 For Example
If AX = 0011000000111001b then AH = 00110000b and AL=001111001b
 Registers are located in CPU,
 Segment Registers
 Used for special purpose pointing at accessible blocks of memory
 It stores any data
 Four segment registers during execution
 CS – Points the segment storing the current program
 DS – Points the segment where variable are defined
 ES – Extra Segment register, depends on the usage of coder
 SS – Points to the segment containing the stack
 Along with general purpose registers, segment registers work
to access any memory value.

13. Flags
 FLAG : The FLAG register is the status register in the Intel 8086
microprocessor that contains the current state of the microprocessor. This
register is 16 bits wide.
 carry flag (CF)- indicates a carry after addition or a borrow after subtraction, also
indicates error conditions.
 parity flag (PF)- is a logic “0” for odd parity and a logic “1” for even parity.
 auxiliary carry flag (AF)- important for BCD addition and subtraction; holds a carry
(borrow) after addition (subtraction) between bits position 3 and 4. Only used for
DAA and DAS instructions to adjust the value of AL after a BCD addition
(subtraction).
 zero flag (ZF)- indicates that the result of an arithmetic or logic operation is zero.
 sign flag (SF)- indicates arithmetic sign of the result after an arithmetic operation.
 overflow flag (OF)- a condition that occurs when signed numbers are added or
subtracted. An overflow indicates that the result has exceeded the capacity of the
machine.

14. Offset
 The offset address in an 8086/8088 is the logical address that
the program "thinks about" when it addresses a location in
memory.
 The Execution Unit (EU or CPU) is responsible for generating
the offset address.
 The Bus Interface Unit (BIU), on the other hand, takes the
offset address and adds it to four times the selected segment
register value in order to determine a real address, which is
now 20-bits in length.

15. Registers in 8086 CPU
 Programmers access various memory locations on combining
BX, SI, DI and BP registers.
 The value in segment registers[CS, DS, SS,ES] is called “segment”
 The value in purpose registers [BX, SI, DI, BP] is called “offset”
 For Example
 DS contains value 1234h and SI contains the value 7890h it can also be recorded as
1234:7890
 Two special purpose registers
 IP – Instruction Register
 Flags Register – determines the current state of the processor

16. Registers in 8086 CPU
 IP works along with CS register and points to the currently
executing instruction
 Flags register is modified by CPU, while performing
mathematical operations

17. Assembler Design Approach
Assembler generates object code.
Role of assembler – Translates the source
code into target code
During this translation process it uses various
table like
Machine Opcode Table
Symbol table
Literal table

18. Assembler Design Approach
Machine Opcode Table
 It holds opcode used by the processor for
different instructions mnemonics
Mnemonic size Opcode
Holds various instructions Size of instructions in bytes
Size of instructions in bytes

19. Symbol Table
 It is data structure used by assmebler
 It keeps into account attributes of the identifier and other
information.
 Attributes : type, value, scope and address
 It also performs the function of book keeping.
 It is composed of multiple word entries in fixed format
Name Value Type

20. One Pass Assembler
Does everything in one pass
Problem: How do we handle forward references?
Could eliminate forward references
easy for data – just define the data areas before they are
referenced
not easy in code – how do we handle selection or loop
statements which have forward jumps?
Two types of one pass assemblers:
produce object code directly in memory
produce object program for later execution

21. One Pass Assembler
 Assembler generates object code instructions as it scans source
program
 If an operand symbol has not yet been defined
 operand address is set to 0 in instruction
 symbol is entered into the symbol table (unless it is already
present)
 entry is flagged to indicate the symbol is undefined
 address of instruction is added to list of forward references
associated with this symbol
 When symbol definition is encountered
 forward reference list is scanned, and proper address is inserted
in any instructions previously generated (in memory)

22. One pass assembler that produces object programs
 Use the same procedure
 When the definition of a symbol is encountered
 if instruction which made the forward reference is still in
memory, then fix it
 if not, the instruction has already been written out in a
Text record, so generate a new Text record with the
correct operand address
 (could also use a modification record)
 The loader will fix up the address field

23. One pass assembler that produces object programs
 Problem : Forward Reference
 It be eliminated by declaring variable before using them
 However, elimination can’t be done easily because sometimes
program needs a forward jump.