Editing MBASIC (section)

== PEEKs, POKEs, and user functions ==
No discussion of BASICs on the [[8-bit computing|8-bit]] computers of the late '70s and early '80s would be complete without mentioning the importance of the [[PEEK and POKE]] functions for directly reading and writing to memory. Since these systems typically had no [[memory protection]], this allowed a programmer to access portions of the operating system, or functions that would not otherwise be available. This also provided opportunities for user programs to hang the system (by accident, usually). For example, a CP/M programmer might use a <code> POKE </code> function to allow BASIC to switch the console device to the serial port, if the system [[BIOS]] supported this. For machines with real-time clocks, a set of <code>PEEK</code> instructions might have been used to access the time.

For more complex operations, MBASIC allowed user-defined functions that could be called from a BASIC program. These were typically placed in a reserved area of memory, or POKEd into string constants, as a series of [[Machine language|machine codes]] ([[opcodes]]). MBASIC also provided hardware <code>INP</code> and <code>OUT</code> instructions that read and wrote directly to the 8080 hardware input/output ports. This could be used to control peripheral devices from a BASIC program if the system hardware permitted.  

Any MBASIC programs that made use of <code>PEEK</code> and <code>POKE</code>, and of machine code user functions, were not portable between machines without modifications.