MBASIC is the Microsoft BASIC implementation of BASIC for the CP/M operating system. MBASIC is a descendant of the original Altair BASIC interpreters that were among Microsoft's first products. MBASIC was one of the two versions of BASIC bundled with the Osborne 1 computer. The name "MBASIC" is derived from the disk file name MBASIC.COM of the BASIC interpreter. MBASIC, like its predecessor family of 8-bit interpreters Microsoft BASIC, were heavily inspired by Digital Equipment Corporation PDP-10's BASIC-PLUS.

MBASIC version 5 required a CP/M system with at least 28 KB of random-access memory (RAM) and at least one diskette drive.

Unlike versions of Microsoft BASIC-80 that were customized by home computer manufacturers to use the particular hardware features of the computer, MBASIC relied only on the CP/M operating system calls for all input and output. Only the CP/M console (screen and keyboard), line printer, and disk devices were available.

MBASIC in the uncustomized form had no functions for graphics, color, joysticks, mice, serial communications, networking, sound, or even a real-time clock function. MBASIC did not fully support the features of the host CP/M operating system, for example, it did not support CP/M's user areas for organizing files on a diskette. Since CP/M systems were typically single-user and stand alone, there was no provision for file or record locking, or any form of multitasking. Apart from these limitations, MBASIC was considered at the time to be a powerful and useful implementation of BASIC.

MBASIC is an 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 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. It was often beneficial to save a program as plain text and edit it with a full featured editor.

Program text, variables, disk buffers and the CP/M operating system itself all had to share the 64 kilobyte address space of the 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 REM 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 " and MERGE 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 loops.

Data could be read and stored to disk as either sequential files (delimited by the CP/M convention of CR/LF at the end of each line) or else as fixed-record-length random access files, which, given a sufficiently determined programmer, could be used to perform database-type record manipulation. The Microsoft Binary Format for floating point numbers was proprietary to the implementation, which meant that data could only be interchanged with other programs using ASCII text representation or else with extensive programming to convert the binary format.