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The X-Philes (2nd Revision)
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The X-Philes Number 1 (1995).iso
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hp48hor1
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checksum.doc
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1995-03-31
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818b
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18 lines
Authors: [Alonzo GARIEPY & Dave Kaffine]
[Alonzo Gariepy:]
The HP 48 has a hardware CRC. Every time you read a nibble from memory the 16
bit CRC at address #00104 is updated. Checksumming a block of memory (as the
BYTES command does) involves zeroing that location and then reading the entire
block of memory. The CRC hardware picks the data right off the bus. Among
other things, this means you can checksum a block of memory at the same time
you move it. Libraries use the same CRC. You must disable interrupts if you
do this from machine language.
[Dave Kaffine:]
There is one important thing to note, however. The CRC that is stored with a
library object does not include the five prolog nibbles 04B20. It starts with
the library length field, just after the prolog.