DistOS 2014W Lecture 4

Discussions on the Alto

CPU, Memory, Disk

CPU

The general hardware architecture of the CPU was biased towards the user, meaning that a greater focus was put on IO capabilities and less focus was put on computational power (arithmetic etc). There were two levels of task-switching; the CPU provided sixteen fixed-priority tasks with hardware interrupts, each of which was permanently assigned to a piece of hardware. Only one of these tasks (the lowest-priority) was dedicated to the user. This task actually ran a virtualized BCPL machine (a C-like language); the user had no access at all to the underlying microcode. Other languages could be emulated as well.

Memory

The Alto started with 64K of 16-bit words of memory and eventually grew to 256K words. However, the higher memory was not accessible except through special tricks, similar to the way that memory above 4GB is not accessible today on 32-bit systems without special tricks.

Task Switching

One thing that was confusing was that they refer to tasks both as the 16 fixed hardware tasks and the many software tasks that could be multiplexed onto the lowest-priority of those hardware tasks. In either case, task switching was cooperative; until a task gave up control by running a specific instruction, no other task could run. From a modern perspective this looks like a major security problem, since malicious software could simply never relinquish the CPU. However, the fact that hardware was first-class in this sense (with full access to the CPU and memory) made the hardware simpler because much of the complexity could be done in software. Perhaps the first hints of what we now think of as drivers?

Graphics, Mouse, Printing

Graphics

A lot of time was spent on what paper and ink provides us in a display sense, constantly referencing an 8.5 by 11 piece of paper as the type of display they were striving for. This showed what they were attempting to emulate in the Alto's display. The authors proposed 500 - 1000 black or white bits per inch of display (i.e. 500 - 100 dpi). However, they were unable to pursue this goal, instead settling for 70 dpi for the display, allowing them to show things such as 10 pt text. They state that a 30 Hz refresh rate was found to not be objectionable. Interestingly, however, we would find this objectionable today--most likely from being spoiled with the sheer speed of computers today, whereas the authors were used to slower performance. The Alto's display took up half the Alto's memory, a choice we found very interesting.

Another interesting point was that the authors state that they thought it was beneficial that they could access display memory directly rather than using conventional frame buffer organizations. While we are unsure of what they meant by traditional frame buffer organizations, it is interesting to note that frame buffer organizations is what we use today for our displays.

Mouse

The mouse outlined in the paper was 200 dpi (vs. a standard mouse from Apple which is 1300 dpi) and had three buttons (one of the standard configurations of mice that are produced today). They were already using different mouse cursors (i.e., the pointer image of the cursor on screen). The real interesting point here is that the design outlined in the paper was so similar to designs we still use today. The only real divergence was the use of optical mice, although the introduction of optical mice did not altogether halt the use of non-optical mice. Today, we just have more flexibility with regards to how we design mice (e.g., having a scroll wheel, more buttons, etc.).

Printer

They state that the printer should print, in one second, an 8.5 by 11 inch page defined with 350 dots/inch (roughly 4000 horizontal scan lines of 3000 dots each). Ironically enough, this is not even what they had wanted for the actual Alto display. However, they did not have enough memory to do this and had to work around this by using things such as an incremental algorithm and reducing the number of scan lines.

Other Interesting Notes

We found it interesting that peripheral devices were included at all.

The author makes a passing mention to having a tablet to draw on. However, he stated that no one really liked having the tablet as it got in the way of the keyboard.

The recurring theme of lack of memory to implement what they had originally envisioned.