CL20 – Building an Assembler
1
HACK Assembly – Manual Assembly – Quick Refresh
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Writing an Assembler
3
Dealing with Variables and Special Symbols
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Dealing with Variables and Special Symbols – Flowchart
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Dealing with LABELS (Branches) – Flowchart
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Comments on Implementation
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Parsing + Translating
Symbolic syntax: dest = comp ; jump
Binary syntax:
1 1 1 a c1 c2 c3 c4 c5 c6 d1 d2 d3 j1 j2 j3 C Instruction
// Assume that current command is
// D = M+1; JGT
String c=parser.comp(); // “M+1”
String d=parser.dest(); // “D”
String j=parser.jump(); // “JGT”
String cc = Code.comp(c); // “1110111”
String dd = Code.dest(d); // “010”
String jj = Code.jump(j); // “001”
String out = “111” + cc + dd + jj;
Trr concatenation
A Instruction
// Assume that current command is @x
String num=parser.var(); //“x”
String addr_data = Code.num(num); //hash table lookup String out = “0” + addr_data;
// Assume that current command is @
String num=parser.num(); //“
String addr_data = num;
String out = “0” + addr_data;
Credit: www.nand2tetris.org 9
Using the Symbol Table
Create a new empty table
Add all the pre-defined symbols to the table
While reading the input, add labels and new variables to the table
Labels: when you see a “(xxx)” command, add the symbol xxx and the address of the next machine language command
• Comment 1: this requires maintaining this running address
• Comment 2: this may need to be done in a first pass
Variables: when you see an “@xxx” command, where xxx is not a number and not already in the table, add the symbol xxx and the next free address for variable allocation
Symbol
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Address
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Credit: www.nand2tetris.org 10
Overall assembler logic
Initialization Of Parser
also initializethe tablewith predefined symbol
Of Symbol Table e
First Pass: Read all commands, only paying attention to labels
and updating the symbol table
Second Pass:
Get the next Assembly Language Command and parse it
For A-commands: Translate symbols to binary addresses
For C-commands: get code for each part and put them together Output the resulting machine language command
Note that
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