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git-svn-id: https://svn.code.sf.net/p/acme-crossass/code-0/trunk@34 4df02467-bbd4-4a76-a152-e7ce94205b78
35 lines
1.5 KiB
Plaintext
35 lines
1.5 KiB
Plaintext
;ACME 0.95
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!ifdef lib_cbm_c64_georam_a !eof
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lib_cbm_c64_georam_a = 1
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; This file contains definitions for accessing the "GeoRAM" RAM expansion and
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; its clones (BBG, BBU, NeoRAM, ...)
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; These units allow access to a single page of memory (256 bytes) visible at
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; address $de00 in i/o space. Writing to registers allows to change which
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; memory page is visible at that location.
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!address {
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; memory page
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georam_page = $de00
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; control registers (write-only, these registers can not be read)
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georam_track = $dffe ; 0..63, i.e. the lower six bits are significant
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georam_sector = $dfff ; 0..31, i.e. the lower five bits are significant
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}
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; these are the official addresses - actually the registers are accessible
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; *everywhere* at $dfxx, but using these locations does not clash with the
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; registers of a Commodore REU.
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; Upgraded units and clones may have more memory, in those cases the registers
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; will have more significant bits.
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; I could have called the registers "row" and "column" instead of track and
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; sector, but the fact that this device was designed with one six-bit register
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; and one five-bit register (instead of one eight-bit register and one
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; three-bit register) tells me that this was meant as an easily programmable
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; RAM disk: A 1541 disk has 35 tracks with (at most) 21 sectors. Numbers in
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; these ranges can be written to the GeoRAM registers without the need to
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; shift bits around.
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; Knowing this is a handy way of remembering the number of significant bits of
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; GeoRAM registers.
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