croft%Safe%su-score@sri-unix.UUCP (02/04/84)
From: Bill Croft <croft%Safe@su-score> Some interesting tidbits that came up at the Apple Macintosh talk last Wednesday at Terman: The 128K byte memory is organized as 64K 16 bit words. During active video, one word can be fetched each microsecond by BOTH the processor and memory. Each microsecond the video takes half the cycle (500 ns) and the processor can take the other. Since only the RAM is shared with video, processor access to ROM can occur at twice this rate (2 megawords per second). This is an important consideration since ROM contains many of the heavily CPU bound code, such as Quickdraw and the OS. The 2 megaword rate can also occur for processor RAM access, when the video is inactive during vertical retrace. It might be a worthwhile option to allow the user to idle the video during lengthy computation (such as a C compilation), if it results in twice the thruput. The disk controller used is the "IWM", Integrated Wozniak Machine, a custom VLSI version of the "classic" Woz design for the Apple II. The disk controller is about the only thing in the system that is not interrupt driven. They were forced to do this because of engineering constraints; they couldn't find a proper FIFO to operate with the IWM. To get around the lack of interrupts, the disk is "polled" during sector IO. During disk IO all other interrupts are turned off for the duration: 12 milliseconds. Fortunately they also poll for serial input during this interval; otherwise they would lose characters. The video screen dot aspect ratio is "square", so that images undergo no distortion in the mapping from memory to display. Unfortunately this is not the case on the Lisa's: the Quickdraw software on those machines must "draw" an ellipse when a circle is desired on the screen; they admitted this was a mistake. Sound is generated by simple pulse width modulation: integration of the digital waveform output results in an inexpensive D/A converter. --Bill Croft, Stanford Medical Center