The Epson HX-20 (also known as the HC-20) is generally regarded as the first laptop computer, announced in November 1981, although first sold widely in 1983. Hailed by BusinessWeek magazine as the "fourth revolution in personal computing", it is generally considered both the first notebook and handheld computer.
The later, more popular TRS-80 Model 100 line, designed by Kyocera, owed much to the design of the HX-20.
When entering Monitor it shows a prompt on the first line, "Trap!" on the second line (if entered via a trap) and the CPU registers on the third and fourth lines as they were right before the Monitor was entered. These registers are A (Accumulator A), B (Accumulator B), X (Index Register), C (Condition Code Register), S (Stack Pointer) and P (Program Counter).
Monitor can be used for reading and writing memory, modifying CPU registers, running code at specific addresses in memory, saving/loading memory to/from a plugin option, etc. This is very useful for debugging programs written in machine code in difference to programs written in the EPSON BASIC programming language.
There's an "expansion unit" which contains sockets for extra RAM and ROMs which can be mapped to various areas in the address space using a DIP switch, even replacing the BASIC interpreter ROMs. Some software distributed as ROMs were designed to be used in the expansion unit.
ROM #0 and #1 are known as the I/O ROMs, handling system reset and providing functions for using the LCD, keyboard, clock, printer, speaker, serial communication, etc. The I/O ROMs are equivalent to the BIOS in modern PCs. ROM #0 also contains the interrupt vector table at FFF0-FFFF. FFFE-FFFF determines what the program counter should be set to on power up or reset. In the standard set of ROMs for the HX-20, this value is E000, the start of ROM #0.
ROM #2 and #3 contains the BASIC interpreter. If the BASIC ROMs are removed from the motherboard, the BASIC option in the main menu will disappear, leaving only MONITOR. This is because ROM #3 contains a program header which is detected by the menu routines. This works the same for all user-created programs, except the program type is different.
An easy fix for replacement batteries is to use four AA cells in a holder secured on the inside. The leads can be easily soldered on to the connector from an original battery. It can also work with a lantern battery on alligator clips.
Category:Laptops Category:Seiko Epson Category:History of computing hardware
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