[COFF] "On designing finger-friendly interactions" article at aresluna.org

[Charles H Sauer (he/him) via COFF]

On 6/21/2026 6:03 PM, Douglas McIlroy via COFF wrote:
> Mention of the bad  old days of half-duplex terminals reminded me of how
> half-duplex custom distorted capacity estimates for full-duplex Multics.
> 
> When Multics first began to work, we at Bell Labs found it could barely
> support three users. Meanwhile, MIT was touting it as already a ten-plus
> user system, based on extrapolating data from CTSS. There were two major
> causes for the discrepancy.
> 
> One cause was that program development bogged the system down; the early
> PL/I compiler was preternaturally slow. In a vicious cycle, slowness
> provided motivation to compile in background, thus putting a double load on
> the system.
> 
> But slow software was not the only thing that caused Multics to choke. "Think
> time" between commands was expected to be the same  on Multics as in CTSS.
> In fact, think time could be zero on Multics because full-duplex
> communication allowed one to type ahead during program execution, whereas
> half-duplex CTSS prevented typing while output from a running command was
> possible. Thus a Multics user could burden the system with more interactive
> commands per minute than a CTSS user could.
> 
> Doug

I wouldn't want to go back, any more than I would want to go back to 
full-duplex on a modem, but half-duplex full screen on a 3270 with 
VM/370 did not seem bothersome to me. Even a 3101 at 1200 baud emulating 
a 3270 full screen half duplex could be an efficient environment.

Charlie

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