No Line-buffer Correction

The user might make a mistake in entering the name and try to correct it by hitting the "backspace" key to erase the mistake. But this does not work. The ascii value of the key 0x08 is included in the string just as any character value. Odd byte values show up (in SPIM) as a small black rectangle.

Most operating system user interfaces allow the user to edit a string (with BS, DEL, arrow and other keys) before it is sent to an application program. The application program asks for a string from the user, but the OS does the actual input, complete with user editing. (This is sometimes called "cooked" input mode).

The SPIM trap hanlder service does not do this. Characters are sent directly to the program as they were typed. If the user hits the backspace character, then that character is placed in the input string. (This is called "raw" input mode).

You could, of course, write your program so that it looks at the raw characters in the buffer and edits the input string. This is a messy task, and often skipped, but should not be.

A famous computer blunder involved a medical radiation therapy device, the Therac 25. The device was controlled by an assembly language program that did raw input for treatment parameters. Backspace and delete characters were added to the input string, not used to correct the string. Unfortunately, when the string was sent to the computer terminal, the terminal responded to backspace and delete characters and showed the operator a string that looked as if it had been edited.

The machine operator would type something like "3<90" (3 back-space 9 0). The terminal would perform the back-space so the operator would see "90". But all four characters were sent to the program. The program would then convert the four "digit" string into an integer. The program did not check for valid input.