Spice series extra RAM
Message #1 Posted by Eric Smith on 8 Apr 2004, 10:03 p.m.
Based on microcode simulations, it appears that some of the Spice series calculators have much more RAM implemented in the hardware than is actually made available to the user. It's not uncommon for a few extra registers to be used for various internal purposes. But the HP-33C appears to have 32 hardware registers, of which 8 are available as user registers and 7 provide the 49 program steps. One is probably Last X, but even if there are a few used for internal status, there are still quite a few unaccounted for.
The register count of 32 was determined by observing the cold-reset memory initialization sequence, which appears to check values in registers 08 and 14 (hex), then zeros all registers from 1F down to 00, then writes initialization constants to registers 08 and 14.
The Spice series, like later Woodstock and Topcat models, uses chips that combine ROM and RAM. Thus the amount of RAM in a Spice is to some extent determined by the ROM size. Possibly the amount of user memory made available was restricted for product positioning purposes.
Simulations of the HP-32E and HP-37E seem to indicate that these models have 32 and 48 registers, respectively. However, this is less conclusive because the initialization in these models is performed by the use of a "clear data registers" instruction that clears an entire 16-register chip in one cycle.
The HP-32E at startup selects and clears the RAM chips at address 10 hex then 00 hex. The HP-37E selects and clears 10, 20, then 00.
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