architecture to the HP 1000/2100 series minicomputer with 16-bit memory address, and an AX and BX general processor register. They ran at a speed comparable to the first IBM PCs. They could draw a mesh of a 3D SIN(X)/X function with no hidden lines over the course of several minutes, a technological breakthrough for the time. Because programs were designed to run from ROM (read only memory) the call subroutine instruction had to be changed because in the HP211x the return location was written in the first location of the subroutine. Instead, another register was created to keep track of return locations on a separate stack area, like more modern processors. This made recursion possible. Some models (e.g. 9835B) used a 32-character 1 line uppercase LED display, which on the one hand might seem limiting, but on the other hand had the same effect as one-line window into a full screen editor which did not become common until the 1980s, with controls to go up or down a line, and cursor left and right, inserting or deleting characters. They powered on ready to do math in "immediate mode", where you would type in an expression such as PRINT 2 + 3, and you would get an answer when you hit enter, without the complication of logging in, or the overhead of maintaining a big computer room and operator. A computer controlled cassette drive using audio cassettes with clear leaders for optical detection of end of tape was used for storage. Random access to a file was by number, but a hard drive could also be attached. The matching line thermal printer was quite fast, printing one horizontal line of dots at once. The speed of a page was faster than later dot matrix printers, and not much worse than modern ink jet printers. HP incorporated thermal printers into many plotting and terminal products later. Programming Although the processor used was only a slight adaptation from that used on HP minicomputers, the system software would be completely redesigned for a computer with its operating system and language system built into read-only memory. BASIC was similar to FORTRAN used by engineers on the HP 1000 minicomputer, but much simpler to use as an interpreted language. Arrow cursor keys were provided which could scroll up and down lines, and interactively insert or delete characters which was unheard of with most CRT or printing