IBM System/23 Datamaster: Difference between revisions
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[[File:IBM 8085 CPU being tested in the Alphatronic P2.jpg|thumb|left|alt=IBM 8085 CPU being tested in the Alphatronic P2|IBM 8085 CPU being tested in the Alphatronic P2]]
As a side note, when studying part of the code, it was discovered that the ROS makes intensive use of the undocumented opcodes of the 8085. This was in part to hinder efforts to reverse-engineer the computer and its firmware and bring compatibility with the System/34 BASIC by emulating some of its registers.
For more specific information about the 8085 microprocessor, [[8085|refer to its article page]].
=== ROM Operating System ===
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Regarding the diagnostics, every single ROS image contains its own reference inside the code. It is still not known at the moment of this writing but it is supposed to be part of the verification algorithm used by procedure PID 1200. Other important aspects to have in mind during the checks are that if the ROM identifier is inverted the memory is faulty. In case it is underlined, usually means that it is not present or detected. Usual causes for this fault are either power supply issues or memory select signals.
Regarding the memories as components, they usually come as Mostek MK36000 or Motorola 68366 and has been reported that in early units the components were Motorola 68766 EPROMs due to delays in supply lines. Nowadays, the Mostek memories are prone to fail and are one of the major causes of failure of this kind of computer. The issue can be solved easily with adapters, in case that the appropriate firmware is provided.
=== RAM Memory ===
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Still, it is known that the official maximum of 128KB is just the maximum for the earlier models of 5322, as later revisions could address up to 256KB by configuring some jumpers in the motherboard. This is possible as the bank control register is implemented with a 74LS670, which is 4 bits wide, giving 16 pages of 16KB each. Additionally, there is a non-banked page which is used among other things as the display memory. For this reason the actual theorical maximum memory for a System/23 is 272KB. Still, it has been hinted that some clones of the system exist and that those expand the width of the page register to eight bits, giving the theorical maximum to 4112KB to them.
The memory boards contain one or two TMS4132 arrays of 9 memories each. Having nine units per bank allows them to have a data byte width of data plus a ninth bit for parity. Both because of the rareness of the format of the TMS4132 and the replacement of the official reference led for years to believe that IBM soldered the memories in piggyback to cope with failure rates when in reality it was a feat of Texas instruments to sell them directly soldered after manufacture of both dies. This kind of memory contains a 16KB bank per package and both packages have a slight difference of the pinout to not have conflict with /CAS and /RAS.
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