DistOS 2014W Lecture 4

Discussions on the Alto

CPU, Memory, Disk

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.

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.