Epson HX-20

The concept behind the HX-20 was first devised in July 1980 by Yukio Yokozawa, who worked for Suwa Seikosha, now the Seiko Epson subsidiary of the Japanese Seiko Group, receiving a patent for the invention. It was announced in 1981 as the HC-20 in Japan, It had a mass-market release in July 1982, as the HC-20 in Japan and as the Epson HX-20 in North America. == Features ==

Epson advertised the HX-20 with a photograph and photo editing of the computer on two facing magazine pages with the headline "Actual size". The Epson HX-20 weighed approximately and featured a full-transit keyboard, rechargeable nickel-cadmium batteries, and a built-in 120 × 32-pixel LCD which allowed 4 lines of 20 characters. (which were essentially an enhanced Motorola 6801), RAM expandable to , two RS-232 ports which was also used by the external floppy disk drive. The battery life of the HX-20 was approximately ours running BASIC and less using the microcassette, printer, or RS-232 port. Data integrity could be preserved in the range. The power supply was rated for . Operating and charging it would tolerate . Data integrity could be preserved at . The HX-20 could be stored between . The later, more popular TRS-80 Model 100 line from Radio Shack, designed by Kyocera, owed much to the design of the HX-20. == Reception ==

In September 1983, the tech magazine BYTE wrote that the HX-20, available in the United States for about a year, had been unsuccessful because of the lack of software or accessories. The review also noted that Epson had included the formerly microcassette drive in the standard configuration, as well as bundling a simple word processor. BYTE praised the printer as "nothing short of amazing" but criticized the lack of an operating system for cassette storage and said that compared to the TRS-80 Model 100's display, "the HX-20 looks primitive". == LCD ==

The LCD is 120×32 pixels and is controlled by six μPD7227 LCD controller ICs, each responsible for 40×16 pixels of the LCD. The μPD7227 uses a serial protocol and has two memory banks for switching between rows 0-7 and 8–15. It features multiple modes, including "Write", "Read", "AND", "OR", and "Character". The "character" mode draws characters from a built-in character map. Each bank is 40 bytes, with bit 6 of the address determining the bank. Even though the address can be up to 127, nothing will happen when trying to access data outside the banks. If the pointer action in a command is set to decrement and the pointer is at 0, the pointer will wrap to 127. == Monitor ==

The Monitor program can be accessed via the main menu on startup by pressing 1, by typing the command "MON" in BASIC, or by causing a trap (that is, writing/reading to/from protected addresses or executing an illegal instruction). In the event of a trap, "Trap!" will be displayed in the Monitor, and the user can use it for debugging. 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 as they were right before the Monitor was entered on the third and fourth lines. These registers are: • A (Accumulator A) • B (Accumulator B) • X (Index Register) • C (Condition Code Register) • S (Stack Pointer) • P (program counter) The 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 contrast to programs written in the EPSON BASIC programming language. Commands == Memory map ==

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. The expansion unit added up to 16 KB of RAM and two ROM sockets. The latter could only be used by switching off the internal BASIC ROMS. == Similar Epson models ==

• HC-80 (Japanese version of the PX-8) • HC-88 (Japanese version of the PX-8) • HX-40 (American version of the PX-4) • HX-45 (American version of the PX-4) • KX-1 • PX-16 (IBM PC compatible portable, cartridges compatible with PX-4) • PX-4 (successor of the HX-20, with larger screen and CP/M compatible like the PX-8) • PX-8 (Geneva) • EHT-30, EHT-40 == See also ==