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June 14, 2021 Archives
Monday, June 14, 2021
CONTEXT: Detecting Polarised Light with the Unaided Eye: Haidinger's Brush
Here is more about Haidinger's brush. Can you see it?
Posted at
14:28
TRACKING WITH CLOSEUPS: How Photographic Film Works, and Why It's Making a Comeback
Posted at
13:48
CONTINUITY: HP 9825 Repair Part 9—Fixing the Memory Expansion Board
With the Hewlett-Packard 9825 laboratory computer booting in “classic” mode, it's time to move on to the memory expansion board, nicknamed “Skoal”, which is one of the supreme sledgehammers ever created to cope with an address space limit. The custom H-P microprocessor used in the computer had 16 address lines, limiting it to a 64 Kb address space, around half of which was occupied by the system firmware. But the customers demanded more! What to do? The designers were not permitted to change the original firmware, which had to remain identical for compatibility with existing software. That ruled out commonly-used schemes such as bank switching (as I used on the Marinchip 9900). So, the Skoal board ascended to a new summit of hackery by overlaying the original address space with additional RAM, then implementing nine rules to decide whether an instruction in the original ROM intended to access RAM or ROM. Eight of the rules handled the most common cases, but for those they didn't catch, there was a ROM with a bit for every instruction address in the original ROM that indicated whether it was accessing the original RAM. These settings were derived from analysis of the code with a logic analyser and the source listing. This is a level of kludge which deserved a Lifetime Achievement Golden Hammer Award, were I still giving them.
Posted at
13:11