Labels are symbolic names for targets of branch instructions

Two-Pass Approach

• generate sequence of instructions with unknown destinations
• once lengths of code are known, resolve all labels
• emit the code

Two main kinds of jumps:

• absolute: indicate address from some fixed position
• relative: indicate address as an increment over current program counter
  • positive increment: jumping forward
  • negative increment: jumping backward

A minimal infrastructure for writing machine code

• allow ASCII names for instructions instead of operation codes
• avoid having to know labels for jumps (what we needed now)
• store constants for strings and other large data (give back the address of that data)

Examples:

• Our cafebabe library enables Scala to be used as an assembler for Java bytecodes
• GNU Assembler
  • take small C file test.c, do 'gcc test.c -S'
  • run the assembler by 'as test.s'