Tutorial 6

Question 1

Read a value in from the user input - READ

Store the read value into x - LOAD_CON 3 (x is offset 3)

Set x on the top of the stack to STORE_FRAME

READ

LOAD_CON 4

STORE_FRAME

Conditional:

Load the value of x LOAD_CON 3

evaluate x < y LESS

LOAD_CON 9# How many instructions (words) we want to skip over

$1+2+1+2+1+2+1$

branch if false BR_FALSE

LOAD_CON 4

LOAD_FRAME (value of y)

LOAD_CON 5 (val of max)

STORE_FRAME

LOAD_CON _____ # Unknown size

BR (else statement, assign max)

//write max

LOAD_CON 5

LOAD_FRAME

WRITE

Question 2

• Only difference to Q1 is that we have a while loop
• Requires branching, except need negative branch value - branch upward

i < 5 → i at offset of 4

LOAD_CON 4

LOAD_FRAME

LOAD_CON 5

LESS

LOAD_CON ____

BR_FALSE // Jump over the while statement

/ while statement

// now want to jump backwards

LOAD_CON ______ (A negative number, jump to the first line)

• When looping backwards, we count the BR command in the offset index count

Question 3

1. Look at the Lexical Rules section, line 131 +
2. PL0.cup Terminal Symbols section, line 68+
3. PL0.cup Grammar rules section, line 169+
4. StatementNode object already added
   • Fill in the code for the static checker for the repeat node
5. implement code part of the repeat node

KW_REPEAT StatementList UNTIL Condition

• Different to a while loop - evaluate before you check the condition
• BR_FALSE {NUMBEROFWORDS}
• In the compiler, Code.SIZE_JUMP_IF_FALSE - gives the size of the BR_FALSE instruction