A machine instruction is a pattern of bits that asks for one machine operation to be executed.
Each MIPS machine instruction is 32 bits (four bytes) long. The three lines after main: call for three machine instructions. The remaining lines consist of information for the assembler and comments (for the human).
For this first program some SPIM options must be set. In the menu bar, click on Simulator then Settings to get the settings dialog. Then click on the MIPS tab.
Select the following options:
| ON | Bare machine |
| ON | Enable Delayed Branches |
| OFF | Enable Mapped I/O |
| OFF | Accept pseudo instructions |
| ON | Enable Delayed Loads |
| OFF | Load exception handler |
These settings simulate a bare machine with no operatings system or other conveniences. Later chapters include some conveniences.
Sometimes you need to close down qtSpim and then start it up again for the settings to take effect.
(Thought Question) Do most actual computers start up as a bare machine?