=> R0 = New bits
R1 = Mask
<= R0 = Old register value
This SWI programs individual bits in the MEMC control register, on machines still supporting Acorn's custom chipsets. Since the MEMC is write-only, this is done via a software copy, maintained by the OS.
Reg = (RegCopy AND NOT R1) OR (R0 AND R1)
To read: R0,R1 = 0
See PRM 1, page 366.
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