6.1.汇编语言与程序

$6.1.$Assemble Program and Assembler

1.Basic assembler logic

• Repeat:

  • Read the next Assembly language command.
  • Break it into the different fields it is composed of.
  • Lookup the binary code for each field.
  • Combine these codes into a single machine language command.
  • Output this machine language command.

• One thing we need to worry about: Symbols:

  • Two cases:

    • Labels: When jumping into a certain part, you give it a name instead of hard core the address. JMP loop
    • Variables: You give a variable name rather than always refer to its exact address in memory. load R1, weight

  • To convert it into concrete address, we use a symbol table:

• To maintain such table:

  • When we first meet a symbol, we need to allocate a new memory location to hold it. For the assembler, it will find the next memory location that's available.
  • When we meet it again, we simply refer to its address in the table.

• One special case: Forward references: We can jump into a label before where it was defined. There are two ways to handle it:

  1. Use a little bit to remember that we've set the labels but don't know where it is yet. We leave it blank until label appears.(more complicated)
  2. In first pass just figure out all address.