Execution – INPUT command (Revised)

Both the INPUT and INPUT PROMPT commands will share routines at run-time. Normally it will not be desirable to call extra functions or check sub-code flags at run-time as this uses execution time (there is overhead calling functions in C/C++). Better if these decisions can be made before hand. But since this is the INPUT command, execution time will not be critical. Therefore, there will be a function for outputting the “? ” (optionally based on a flag argument) and reading the input from the keyboard.

Instead of just pushing the location of the input get code to the evaluation stack (so that it can be re-executed upon an error), a flag for whether there is a prompt expression and a flag whether the prompt is a temporary string will be pushed along side the location. In other words, there will be an input stack item structure containing the location, prompt flag and temporary flag. So when the Input or InputPrompt code is executed, they will have access to whether there was a prompt string (that needs to be popped) and whether it is a temporary string (that needs to be deleted).

But is it necessary to know there is a prompt since Input will know that there is no prompt and InputPrompt will know there is a prompt (though it still needs to know if the prompt is a temporary string). Both Input and InputPrompt will need to check for an EOL in the input buffer, process the assignments on the stack, pop the location entry from the stack, and check if the cursor should be kept on the same line.

Since the temporary flag needs to be carried from the InputPromptStr and InputPromptTmp codes (why not also carry the prompt flag), and both Input and InputPrompt share a lot of the same work, both will share the same run-time routine. This is verses having three routines (one for Input, one for InputPrompt, and one for the common work). The single run-time routine will check for a prompt (which will be popped from the stack) and if it is temporary (which will be deleted).

The InputGet code will push the location with no prompt flag, and then call the get input routine with the question flag argument set. The InputPromptStr will push the location with the prompt flag set and no temporary flag, and then call the get input routine with the question flag argument set if the Question sub-code flag is set. The InputPromptTmp will push the location with the prompt flag and temporary flag set, and then call the get input routine with the question flag argument set if the Question sub-code flag is set. Now back to the implementation of the INPUT statement translation...