Editing MBASIC (section)

=== Language system ===
MBASIC is an [[BASIC interpreter|interpreter]]. Program source text was stored in memory in tokenized form, with BASIC keywords replaced by one-byte tokens which saved memory space and speeded execution. Any line prefixed with a line number was stored as program text; BASIC statements not prefixed with a line number were executed immediately as commands. Programs could be listed on the screen for editing, or saved to disk in either a [[data compression|compressed]] binary format or as plain [[ASCII]] text. Every source line was identified with a number, which could be used as the target of a [[GOTO]] or [[GOSUB]] transfer. Only line editing commands were provided.<ref>CP/M products providing full-screen editing support required their own installation routines to customize the software for the specific [[computer terminal]] used as the system console. No support was provided within CP/M to standardize terminal capabilities.</ref> It was often beneficial to save a program as plain text and edit it with a full featured editor.

Program text, [[Variable (computer science)|variables]], [[disk buffer]]s and the CP/M operating system itself all had to share the 64 kilobyte address space of the [[Intel 8080|8080]] processor. Typically when first starting MBASIC there would be less than 32 kB memory available for programs and data, even on a machine equipped with a full 64 kilobytes of RAM. Comment lines, prefixed with the <code> REM </code> keyword or an apostrophe, could be placed in the program text but took up valuable memory space, which discouraged BASIC users from fully documenting their code. To allow larger and more complex programs to be run, later versions of MBASIC supported functions that allowed portions of program text to be read in and executed under program control (the "[[chain loading|<code> CHAIN </code>]]" and <code> MERGE </code> statements). No support for "shell" command execution was provided, though this functionality could be duplicated by a determined programmer.

A particular advantage of MBASIC was the full-text error messages provided for syntax and run-time errors. MBASIC also had a "trace" function that displayed line numbers as they were executed. While this occupied the same screen space as normal program output, it was useful for detecting conditions such as [[endless loop]]s.