Embeded System II - Assembly Language

 An assembly language is a low-level programming language for computers, microprocessors, micro controllers, 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.

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 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.

title   Hello World Program                             (hello.asm)
; This program displays "Hello, World!"

dosseg
.model small
.stack 100h

.data
hello_message db 'Hello, World!',0dh,0ah,'$'

.code
main  proc
      mov    ax,@data
      mov    ds,ax

      mov    ah,9
      mov    dx,offset hello_message
      int    21h

      mov    ax,4C00h
      int    21h
main  endp
end   main

 Assembly Language Tutorial #1 Introduction 

Historical perspective

Assembly languages were first developed in the 1950s, when they were referred to as second generation programming languages. For example, SOAP (Symbolic Optimal Assembly Program) was a 1957 assembly language for the IBM 650 computer. Assembly languages eliminated much of the error-prone and time-consuming first-generation programming needed with the earliest computers, freeing programmers from tedium such as remembering numeric codes and calculating addresses. They were once widely used for all sorts of programming. However, by the 1980s (1990s on microcomputers), their use had largely been supplanted by high-level languages[citation needed], in the search for improved programming productivity. Today, although assembly language is almost always handled and generated by compilers, it is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems.

Historically, a large number of programs have been written entirely in assembly language. Operating systems were almost exclusively written in assembly language until the widespread acceptance of C in the 1970s and early 1980s. Many commercial applications were written in assembly language as well, including a large amount of the IBM mainframe software written by large corporations. COBOL and FORTRAN eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 90s.

Most early microcomputers relied on hand-coded assembly language, including most operating systems and large applications. This was because these systems had severe resource constraints, imposed idiosyncratic memory and display architectures, and provided limited, buggy system services. Perhaps more important was the lack of first-class high-level language compilers suitable for microcomputer use. A psychological factor may have also played a role: the first generation of microcomputer programmers retained a hobbyist, "wires and pliers" attitude.

In a more commercial context, the biggest reasons for using assembly language were minimal bloat (size), minimal overhead, greater speed, and reliability.

Typical examples of large assembly language programs from this time are IBM PC DOS operating systems and early applications such as the spreadsheet program Lotus 1-2-3, and almost all popular games for the Atari 800 family of home computers. Even into the 1990s, most console video games were written in assembly, including most games for the Mega Drive/Genesis and the Super Nintendo Entertainment System[citation needed]. According to some industry insiders, the assembly language was the best computer language to use to get the best performance out of the Sega Saturn, a console that was notoriously challenging to develop and program games for .The popular arcade game NBA Jam (1993) is another example. On the Commodore 64, Amiga, Atari ST, as well as ZX Spectrum home computers, assembler has long been the primary development language. This was in large part because BASIC dialects on these systems offered insufficient execution speed, as well as insufficient facilities to take full advantage of the available hardware on these systems. Some systems, most notably Amiga, even have IDEs with highly advanced debugging and macro facilities, such as the freeware ASM-One assembler, comparable to that of Microsoft Visual Studio facilities (ASM-One predates Microsoft Visual Studio).

The Assembler for the VIC-20 was written by Don French and published by French Silk. At 1639 bytes in length, its author believes it is the smallest symbolic assembler ever written. The assembler supported the usual symbolic addressing and the definition of character strings or hex strings. It also allowed address expressions which could be combined with addition, subtraction, multiplication, division, logical AND, logical OR, and exponentiation operators.

List of Interrupts

13           INT 00 through INT 10/BE

14           INT 10/BF through INT 15/0F

15           INT 15/10 through INT 15/E7

16           INT 15/E8 through INT 1A/B0

17           INT 1A/B1 through INT 1F

18           INT 20 through INT 21/43

19           INT 21/44 through INT 21/5E

20           INT 21/5F through INT 21/E2

21           INT 21/E3 through INT 21/F1

22           INT 21/F2 through INT 25

23           INT 26 through INT 2F/15

24           NT 2F/16 through INT 2F/79

25           INT 2F/7A through INT 2F/D9

26           INT 2F/DA through INT 50

27           INT 51 through INT 61

28           INT 62 through INT 6A

29           INT 6B through INT 91

30           INT 92 through INT FF

Example Assembly Code: Hello Word

title   Hello World Program                             (hello.asm)

; This program displays "Hello, World!"

dosseg

.model small

.stack 100h

.data

hello_message db 'Hello, World!',0dh,0ah,'$'

.code

main  proc

      mov    ax,@data

      mov    ds,ax

     mov    ah,9

      mov    dx,offset hello_message

      int    21h

      mov    ax,4C00h

      int    21h

     main  endp

     end   main

Example Assembly Code: A complete program to display the letter ‘a‘on the screen

; prog1.asm: displays the character ‘a’ on the screen
; Author: Joe Carthy
; Date: March 1994
.model small
.stack 100h
.code
start:
mov dl, ‘a’ ; store ascii code of ‘a’ in dl
mov ah, 2h ; ms-dos character output function
int 21h ; displays character in dl register
mov ax, 4c00h ; return to ms-dos
int 21h
end start