The instruction to switch off the vOverlay bit can be found at location 0xce in the Mac Plus ROM listing: https://www.bigmessowires.com/rom-adapter/plus-rom-lis

The instruction to switch off the vOverlay bit can be found at location 0xce in the Mac Plus ROM listing: https://www.bigmessowires.com/rom-adapter/plus-rom-listing.asm https://www.bigmessowires.com/rom-adapter/plus-rom-listing.a... The VIA vOverlay bit is connected to one of the PALs (early programmable logic chips) in the Mac, the BMU1. The relevant equations to create the RAM and ROM chip selects are (from http://www.retro.co.za/ccc/mac/ReverseEngineering/PALs.html http://www.retro.co.za/ccc/mac/ReverseEngineering/PALs.html): /ROM := /A23 * A22 * /A21 * /AS ; 0x40 0000 - 0x5F FFFF + /A23 * /A22 * /A21 * /AS * OVERLAY ; 0x00 0000 - 0x1F FFFF when OVERLAY enabled + A23 * /A22 * /AS ; 0x80 0000 - 0xBF FFFF + A23 * /A21 * /AS ; 0x80 0000 - 0x9F FFFF and 0xC0 0000 - 0xDF FFFF /RAM := /A23 * /A22 * /A21 * /AS * /OVERLAY ; 0x00 0000 - 0x1F FFFF + /A23 * A22 * A21 * /AS * OVERLAY ; 0x60 0000 - 0x7F FFFF when OVERLAY is enabled Here you can see that the OVERLAY bit coming from the VIA is 1. used to additionally map the ROM at address 0 and 2. used to map RAM to 0x600000 (when OVERLAY is enabled) and to 0x000000 (when OVERLAY is disabled).