Snapshot of this Topic
01. Introduction
02. Role of Assemblers
03. Example Program
04. Directives
05. Functions
06. Key Points

• It is Software that translates assembly language into machine language. Contrast with compiler, which is used to translate a high-level language, such as COBOL or C, into assembly language first and then into machine language.
• System software helps run the computer hardware and computer system. It includes operating systems, device drivers, diagnostic tools, servers, windowing systems, utilities, language translator, data communication programs, data management programs and more.
• The purpose of systems software is to insulate the applications programmer as much as possible from the details of the particular computer complex being used, especially memory and other hardware features, and such accessory devices as communications, printers, readers, displays, keyboards, etc.

Introduction to Assemblers

• Fundamental functions
• translating mnemonic operation codes to their machine language equivalents
• assigning machine addresses to symbolic labels

• Machine dependency
• different machine instruction formats and codes

• Data transfer (RD, WD)
• a buffer is used to store record
• buffering is necessary for different I/O rates
• the end of each record is marked with a null character (0016)
• the end of the file is indicated by a zero-length record
• Subroutines (JSUB, RSUB)
• RDREC, WRREC
• save link register first before nested jump

• Pseudo-Instructions
• Not translated into machine instructions
• Providing information to the assembler
• Basic assembler directives
• START
• END
• BYTE
• WORD
• RESB
• RESW
  

• Header

Col. 1                                                   H
Col. 2~7                                               Program name
Col. 8~13                                             Starting address (hex)
Col. 14-19                                            Length of object program in bytes (hex)

• Text

Col.1                                                    T
Col.2~7                                                Starting address in this record (hex)
Col. 8~9                                               Length of object code in this record in bytes (hex)
Col. 10~69                                           Object code (69-10+1)/6=10 instructions

• End

Col.1                                                    E
Col.2~7                                                Address of first executable instruction (hex)
                                                                                    (END program_ name)