Zapple



The Zapple Monitor was a Z-80 (8 bit CPU) ROM based I/O handler and debugging tool that was developed in 1976 by Roger W. Amidon for his company "Technical Design Labs". The basic design came from Roger's original "Apple Monitor", an 8080 (8 bit CPU) version of the monitor, originally written and designed for use on the Altair 8080 personal computers.

To understand what this is all about requires some knowledge of the computer industry at the dawn of the personal computer revolution. If you had a CPU with only some RAM memory connected to it, like in the original Altair computer, it was somewhat useless until you first had a list of "instructions" for the CPU to perform. In the beginning, the method used to input the first set of instructions was the use of 24 switches and some buttons. 8 of the switches were connected to the "Data" lines of the computer, and the other 16 switches were connected to the "Address" lines of the computer. One of the buttons was used to force the data value from the 8 switches to be copied into RAM at the address set by the 16 address switches. Usually, there was a seperate "I/O" (Input/Output) device also attached to the computer which had the ability of automatically feeding additional instructions into the "RAM". This was often a punched paper tape reader, or sometimes a digital tape player. However, since it took some CPU intructions to run this device, those instructions had to be entered by hand. This technique was called "Boot Strapping". This is where we get the term "Booting" a computer. In 1976, with the availability of 2K byte ROM's, it was possible to eliminate the first step in this "Boot" process. The top 2K bytes of CPU memory contained a ROM that had a set of intructions to allow the reading of the tape device, which made loading the program you wished to run a whole lot easier. In addition, other abilities were built into this ROM. Keyboard input routines, display output routines, memory dump routines, memory modification routines, CPU register dumps, breakpoints, etc. could all be included in this 2K ROM. The first one was designed by Roger W. Amidon for the Altair Computer. It was named the "Apple Monitor". The reason for the name was Roger's love of the Beatles and their music. When they came out with the "Apple" record label, it seemed neat to name this monitor the "Apple Monitor" in honor of them. Later, when the Z-80 CPU (which was fully 8080 compatible) was developed, a special version of the Apple monitor was written, taking full advantage of the enhanced intruction set and features of the Z-80 CPU. In order to distinguish between the two monitors, a "Z" was added, thus the "Zapple Monitor" was born. Estimates are that over 200,000 Zapple monitors were in use between 1976 and 1986. Indeed, some are still in use even today.

NOTE: I really do plan to add other items to this web site. Next will be the complete annotated source code for the original Zapple monitor. As time permits...

(More to come...)

Roger W. Amidon

(609) 466-2092


A pioneer in the personal computer revolution since 1972.

Consulting since 1986.

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