Significance of the IBM 5100 CRT


The IBM S/360 and S/3, like most computers at that time, had no “screen.” To see the output of a program, or observe the state of the system, the pre-1970s computers used either a grid of LED lights to indicate a single-bit state, or a line printer (see here). You could only print-forward, there was no concept of “clearing the screen” or writing to specific locations of the screen (because there was no screen). Adding the CRT to the IBM 5100 was like a kind of reusable-electronic-paper, where you could scroll both up and down, and interactively conduct work without needing a “screaming” printer (or ink).

While IBM didn’t originate the idea of a CRT or the idea of coordinated character-generator to render a consistent multi-row text-screen, the inclusion of this capability in the IBM 5100 helped evolve the thinking that this should be a standard component of a personal computer. The Common ROS (ROM) helped manage how the results of the emulated S/360 or S/3 that were suited to go to a printer, would instead be routed to the CRT screen.

Since neither S/360 or S/3 had the concept of clearing the screen or writing to arbitrary positions of the screen, these actions can’t be performed in APL or BASIC on the IBM 5100 (because the systems they are emulating can’t do these functions). That is, there is no “CLS” command in the IBM 5100 BASIC to simply clear the screen. But the system does have a concept of Devices, and the Executive ROS (a kind of early ROM BIOS) has a feature to Alter addresses and enter PALM machine code directly into RWS (RAM) — using this, users could invoke commands (in machine code) to do functions like clearing the screen.

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