The Zapple Monitor was a

firmware In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, h ...

-based product developed by Roger Amidon at Technical Design Laboratories (also known as ''TDL''). TDL was based in

Princeton, New Jersey The Municipality of Princeton is a Borough (New Jersey), borough in Mercer County, New Jersey, United States. It was established on January 1, 2013, through the consolidation of the Borough of Princeton, New Jersey, Borough of Princeton and Pri ...

, USA in the 1970s and early 1980s.Advertisement: XITAN Alpha 1 and Alpha 2 from Technical Design Labs
Published 1977, From Volume 1 Issue 1 of ROM Magazine. The ''Zapple'' monitor was a primitive

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

which could be expanded and used as a Basic Input/Output Services (

BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...

)

8080 The Intel 8080 is Intel's second 8-bit microprocessor. Introduced in April 1974, the 8080 was an enhanced successor to the earlier Intel 8008 microprocessor, although without binary compatibility.'' Electronic News'' was a weekly trade newspa ...

- and

Z80 The Zilog Z80 is an 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be software-compatible with the Intel 8080, offering a compelling altern ...

-based computers. Much of the functionality of Zapple would find its way into applications like 'Debug' in

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few op ...

. Zapple commands would allow a user to examine and modify memory, I/O, execute software (Goto or Call) and had a variety of other commands. The program required little in the way of then-expensive

read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...

(ROM) or

RAM Ram, ram, or RAM most commonly refers to: * A male sheep * Random-access memory, computer memory * Ram Trucks, US, since 2009 ** List of vehicles named Dodge Ram, trucks and vans ** Ram Pickup, produced by Ram Trucks Ram, ram, or RAM may also ref ...

. An experienced user could use Zapple to test and debug code, verify hardware function, test memory, and so on. A typical command line would start with a letter such as 'X' (examine memory) followed by a

hexadecimal Hexadecimal (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...

word (the memory address 01AB) and

nter The Northern Territory National Emergency Response, also known as "The Intervention" or the Northern Territory Intervention, and sometimes the abbreviation "NTER" (for Northern Territory Emergency Response) was a package of measures enforced by ...

or pace After this sequence the content of the memory location would be shown Fand the user could enter a hexadecimal

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...

0to replace the contents of the address, or hit paceor

to move to the next address 1AB An experienced user could enter a small program in this manner, entering machine language from memory. Because of the simple structure of the program, consisting of a vector table (one for each letter) and a small number of subroutines, and because the source code was readily available, adding or modifying Zapple was straightforward. The dominant operating system of the era,

CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/Intel 8085, 85-based microcomputers by Gary Kildall of Digital Research, Dig ...

, required the computer manufacturer or hobbyist to develop hardware specific BIOS. Many users tested their BIOS subroutines using Zapple to verify, for example, a

floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...

track seek command, or read sector command, etc., was functioning correctly by extending Zapple to accommodate these operations in the hardware environment. The general structure of Zapple lives on in the code of many older programmers working on embedded systems as it provides a simple mechanism to test the hardware before moving to more advanced user interfaces.

References

External links

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TDL Apple and Zapple monitor
History of software {{operating-system-stub