I somewhat recently read j. b. crawford's ipmi, where in a part crawford talks about how old servers of the late 80s and 90s (Unix and otherwise) often had various features for management like serial consoles. What makes something an old school 80s and 90s Unix server and why they died off is an interesting topic I have views on, but today I want to mention and cover a much smaller one, which is that this sort of early boot environment and low level management system was generally also found on Unix workstations.

By and large, the various companies making both Unix servers and Unix workstations, such as Sun, SGI, and DEC, all used the same boot time system firmware on both workstation models and server models (presumably partly because that was usually easier and cheaper). Since most workstations also had serial ports, the general consequence of this was that you could set up a 'workstation' with a serial console if you wanted to. Some companies even sold the same core hardware as either a server or workstation depending on what additional options you put in it (and with appropriate additional hardware you could convert an old server into a relatively powerful workstation).

(The line between 'workstation' and 'server' was especially fuzzy for SGI hardware, where high end systems could be physically big enough to be found in definite server-sized boxes. Whether you considered these 'servers with very expensive graphics boards' or 'big workstations' could be a matter of perspective and how they were used.)

As far as the firmware was concerned, generally what distinguished a 'server' that would talk to its serial port to control booting and so on from a 'workstation' that had a graphical console of some sort was the presence of (working) graphics hardware. If the firmware saw a graphics board and no PROM boot variables had been set, it would assume the machine was a workstation; if there was no graphics hardware, you were a server.

As a side note, back in those days 'server' models were not necessarily rack-mountable and weren't always designed with the 'must be in a machine room to not deafen you' level of fans that modern servers tend to be found with. The larger servers were physically large and could require special power (and generate enough noise that you didn't want them around you), but the smaller 'server' models could look just like a desktop workstation (at least until you counted up how many SCSI disks were cabled to them).

Sidebar: An example of repurposing older servers as workstations

At one point, I worked with an environment that used DEC's MIPS-based DECstations. DEC's 5000/2xx series were available either as a server, without any graphics hardware, or as a workstation, with graphics hardware. At one point we replaced some servers with better ones; I think they would have been 5000/200s being replaced with 5000/240s. At the time I was using a DECstation 3100 as my system administrator workstation, so I successfully proposed taking one of the old 5000/200s, adding the basic colour graphics module, and making it my new workstation. It was a very nice upgrade.