[HN Gopher] Digital echoes: open bus behavior on the compact Mac...
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       Digital echoes: open bus behavior on the compact Macintosh
        
       Author : zdw
       Score  : 27 points
       Date   : 2025-11-17 03:46 UTC (5 days ago)
        
 (HTM) web link (thomasw.dev)
 (TXT) w3m dump (thomasw.dev)
        
       | retrac wrote:
       | > The CPU (the Motorola 68000) initiates all communication in the
       | case of the Macintosh
       | 
       | Minor nitpick but the video controller has direct access to RAM
       | and when the visible part of the display is being drawn by the
       | video controller, it does cycle-stealing DMA that preempts the
       | CPU.
       | 
       | You can read in depth about it from Steve Chamberlain, the
       | designer of an FPGA low level clone of the Mac Plus here: [0]
       | 
       | By the way but this why the compact Macs have CPU performance of
       | about a 6 MHz 68000 when clocked at almost 8 MHz. About half the
       | time the display is being redrawn. And about half the memory
       | bandwidth is used up at those times.
       | 
       | Bit of a tangent but the Apple II used a similar approach for its
       | video system. As the article notes, the 68K has a prefetch queue.
       | So it's always reading ahead which is what you get from reading
       | an open bus. But the 6502 is a simple CPU without that fancy
       | prefetch queue, so if you read a non-extant address on the Apple
       | II you get what the video system just read and drew in the
       | previous half-cycle. It's possible to sync up the CPU with the
       | video system this way; something otherwise impossible on the
       | Apple II. [1]
       | 
       | [0]
       | https://www.bigmessowires.com/2011/08/25/68000-interleaved-m...
       | 
       | [1] http://www.deater.net/weave/vmwprod/megademo/vapor_lock.html
        
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       (page generated 2025-11-22 23:00 UTC)