I am working on the disassembly and commenting of the 98228A disk ROM for the 9825T, and my disassembler flagged two instructions as invalid. They are used inside a routine that copies blocks of words from various banks of the ROM into low RAM. The first, bit pattern 070113, is used immediately after a dir (disable interrupt) instruction. The second, bit pattern 070117, is used immediately before an eir (enable interrupt) instruction. The “invalid” instructions do not match any instructions described in the 9825A patent, nor are present in the 9835 or 9845 assemblers instruction descriptions. From the surrounding code, it doesn’t appear that these instructions reference any of the user visible CPU registers, but are used in some way that enhances the effect of dir/eir and ensures the block copy is not interfered with.
Anyone have any ideas? Possibly a DMA request ignore/resume pair?
