TC1 postscript mark II

If one postscript is good, two are even better. We’ve added this second variation on a theme to demonstrate some different ways of doing things. Much of the program’s structure is the same as before. The features are as follows:

• Direct execution (revisited)
• The ability to avoid different mnemonics (e.g., ADD and ADDL) for the same basic operation

The principal enhancement is the way to handle instructions and decode them. In TC1, we use a 4-bit code to define the structure of each instruction in terms of its parameters. When a mnemonic is looked up in the dictionary, it returns a code giving the required parameters.

One feature (problem?) with TC1 is that we have different mnemonics for variations on an instruction, for example, ADD and ADDL. The suffix L tells the assembler that a literal operand (rather than a register number) is required. In this example, we avoid different instruction formats and use a single mnemonic by...