1. Processor Fundamentals
Assembly Language

2. Learning Objectives
Show understanding of the relationship between assembly language and machine code, including symbolic and absolute addressing, directives and macros.
Describe the different stages of the assembly process for a ‘two-pass’ assembler for a given simple assembly language program
Trace a given simple assembly language program

3. Low Level Programming Languages / Paradigm (method of programming)
Instructions are either in machine code or they are one to one with machine code e.g. assembly language
See next two slides.
Are “close to the hardware” as they provide little or no abstraction from a computer's instruction set architecture (see Fetch-Decode-Execute-Reset Cycle).
Meaning that each low level language instruction is one operation the processor executes which can only be one of 3 types: Arithmetic / Jump / Control.
Low-level language programs written for 1 computer will not necessarily work on another using a different processor, chip or architecture.

4. Assembly Languages
Use of mnemonics.

5. Absolute Addressing Refers directly to a memory location e.g. 2011.
Advantage:
Simple.
Disadvantages:
Difficult to see the meaning of the data in location 2011.
The data may not be able to be located at 2011 because another program is already using that location.
Absolute addressing produces non-relocatable code.

6. Symbolic Addressing
e.g.
// Computes sum=
00 LDM 0
01 STO Index
02 STO Sum
Loop:
03 LDD Index
04 CMP 101
05 JPE End
06 INC ACC
07 STO Index
08 LDD Sum
09 ADD Index
10 STO Sum
11 JMP Loop
End:
12 End
…..
1024 Index:
1025 Sum:
Define a symbol for the data item or location instead of referring to an absolute location,
Labels / Variables: .
Label various locations in the program with symbols.
For example, one can declare the label loop to refer to the beginning of a certain code segment. Other commands in the program can then jump to loop, either conditionally or unconditionally.
Assign symbolic variable names to memory addresses.
Predefined Symbols:
See slide 4.
Forward References
Forward References
Forward References

7. Directives
e.g.
// Computes sum=
00 LDM 0
01 STO Index
02 STO Sum
Loop:
03 LDD Index
04 CMP 101
05 JPE End
06 INC ACC
07 STO Index
08 LDD Sum
09 ADD Index
10 STO Sum
11 JMP Loop
End:
12 End
…..
1024 Index:
1025 Sum:
Provide information to the assembler but do not generate any code.

8. Macros
Many programs contain sequence of instructions which are repeated in identical form.
A macro facility permits us to attach a name to this sequence and to use this name in its place.
e.g.
Start of definition MACRO
Macro name CNTR (for example)
Sequence to be abbreviated - -
END of a Macro definition MEND

9. Two-Pass Assembler First pass: Second pass:
The assembler builds a symbol table and generates no code.
Second pass:
The assembler replaces each symbol with its corresponding meaning (numeric address) and generates the final binary code.
e.g.
// Computes sum=
00 LDM 0
01 STO Index
02 STO Sum
Loop:
03 LDD Index
04 CMP 101
05 JPE End
06 INC ACC
07 STO Index
08 LDD Sum
09 ADD Index
10 STO Sum
11 JMP Loop
End:
12 End
…..
1024 Index:
1025 Sum:
00 LDM 0
01 STO 1024
02 STO 1025
03 LDD 1024
04 CMP 101
05 JPE 12
06 INC ACC
07 STO 1024
08 LDD 1025
09 ADD 1024
10 STO 1025
11 JMP 3
12 End
…..
1024
1025
Machine Code:
Symbol Table
Index
1024
Sum
1025
Loop 3
End 12
Forward References

10. Assembly Languages
Use of mnemonics and names/labels (instead of addresses) for locations in memory.
Each assembly instruction represents a single machine instruction which means that it is fairly easy to translate a program written in assembly language to machine code.
Writing programs in assembly language, although easier than using machine code, is still tedious and takes a long time.

11. Plenary
Explain the relationship between assembly languages and machine code.

12. Assembly Languages
Use of mnemonics and names/labels (instead of addresses) for locations in memory.
Each assembly instruction represents a single machine instruction which means that it is fairly easy to translate a program written in assembly language to machine code.
Writing programs in assembly language, although easier than using machine code, is still tedious and takes a long time.

13. Plenary
Describe how an assembler produces machine code from assembly language.

14. Two-Pass Assembler First pass: Second pass:
The assembler builds a symbol table and generates no code.
Second pass:
The assembler replaces each symbol with its corresponding meaning (numeric address) and generates the final binary code.
e.g.
// Computes sum=
00 LDM 0
01 STO Index
02 STO Sum
Loop:
03 LDD Index
04 CMP 101
05 JPE End
06 INC ACC
07 STO Index
08 LDD Sum
09 ADD Index
10 STO Sum
11 JMP Loop
End:
12 End
…..
1024 Index:
1025 Sum:
00 LDM 0
01 STO 1024
02 STO 1025
03 LDD 1024
04 CMP 101
05 JPE 12
06 INC ACC
07 STO 1024
08 LDD 1025
09 ADD 1024
10 STO 1025
11 JMP 3
12 End
…..
1024
1025
Machine Code:
Symbol Table
Index
1024
Sum
1025
Loop 3
End 12
Forward References