1. Computer Science 210 Computer Organization
Building an Assembler
Part IV: Grammars and Syntax Analysis

2. The Second Pass Text file Line stream Token stream
Tools
Opcode table
Text file
Line stream
CharacterIO
Scanner
Token stream
First Pass
Symbol table
Second Pass
Source program listing,
error messages
(file and/or terminal)
Sym file
Bin file

3. 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

4. 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

5. Example Types of Symbols
Terminals are "ADD" and "x3000"
Non-terminals are label and string-literal
Metasymbols are = and |

6. 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

7. 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

8. For Wednesday
Code Generation