Computer Science 210
Computer Organization
Building an Assembler
Part IV: Grammars and Syntax Analysis
The Second Pass
Tools
Opcode table
Line stream
Text file
CharacterIO
Scanner
Token stream
First Pass
Source program listing,
error messages
(file and/or terminal)
Symbol table
Sym file
Second Pass
Bin file
Syntax Analysis
• Examine each token in a line of code and
verify that the line obeys the syntax rule for
a type of instruction
• An instruction’s type is determined by its
leading token and possible other tokens
• Specify all the syntax rules in a grammar
EBNF Grammar
• Extended Backus-Naur Form
• Contains rules and three types of symbols
– Terminal symbols: can appear in code
– Non-terminal symbols: name phrases
– Metasymbols: used to form the rules
Example Types of Symbols
• Terminals are "ADD" and "x3000"
• Non-terminals are label and string-literal
• Metasymbols are = and |
Some Example Rules
add-or-and-ins = ( "ADD" | "AND" ) register "," register ","
( register | integer-literal )
blkw-ins = ".BLKW" integer-literal
br-ins = br-opcode label
=
means "is defined as"
" " enclose literal items
[ ] enclose optional items
{ } enclose items that can appear zero or more times
( ) group items together as a unit
|
indicates a choice
The Complete Grammar (in Grammar.txt)
• Contains rules for all the syntactic forms
• Mentions most of the tokens, except for the
integers and labels, but specifies rules for
constructing and recognizing those
For Wednesday
Code Generation