Editing IBM 5100 (section)

==Programming languages==
The 5100 was available with [[APL programming language|APL]], [[BASIC programming language|BASIC]], or both programming languages.{{r|byte197512}} At the time of introduction, APL was generally available only on mainframe computers, and most desktop sized computers such as the [[Wang 2200]] or [[HP 9830]] offered only BASIC. As a desktop computer offering APL, the 5100 competed with, and indeed may have been inspired by, the earlier [[MCM/70]].<ref name="Stachniak2011">{{cite book
    |author=Zbigniew Stachniak|title=Inventing the PC: The MCM/70 Story|url=https://books.google.com/books?id=cyWOA2FED7EC&pg=PA140|year=2011|publisher=McGill-Queen's Press - MQUP|isbn=978-0-7735-8146-3|pages=140–}}</ref>

Machines that supported both languages provided a toggle switch on the front panel to select the language. On the 5100's front panel, it was the third toggle from the left: up for APL, down for BASIC.<ref name=IBM5100.MoData75>{{cite magazine |magazine=[[Modern Data (magazine)|Modern Data]]
    |pages=50–55 |title=The IBM 5100 Portable Computer |date=October 1975}}</ref>

When the engineers at IBM asked one beta tester, Donald Polonis, for his analysis, he commented that if folks had to learn APL to use it, the IBM 5100 would not make it as a personal computer. He tried to impress the fact that a personal computer had to be easy to use to be accepted.<ref>{{cite book
   |url=https://books.google.com/books?id=lB4PAwAAQBAJ&pg=PA39
   |title=Games vs. Hardware. The History of PC video games: The 80s
   |quote=When the engineers at IBM asked one beta tester, Donald Polonis ...
   |author=Bogdan Ion Purcaru |year=2014}}</ref> Presumably, the special APL character set and APL keyboard were the primary obstacles to newcomers learning APL easily.  APL had powerful features for manipulating data as [[array data structure|vectors]] and [[matrix (mathematics)|matrices]], while the competing [[HP 9830]] had to offer language extensions on an add-on ROM for matrix operations.

Although not meant for regular users, the maintenance manual described a keyboard sequence to switch the 5100 into a maintenance mode. In this mode it was possible to read and write directly in RAM memory, video memory, CPU registers, interrupt vectors, clock counter, etc., using hexadecimal codes equivalent to assembly language. This allowed writing sophisticated programs directly into RAM. As this mode was a single-user system effectively running without an operating system, a determined user could manage the memory space and write stable multi-tasking programs using interrupts.