Main ROM Management Feature
Main ROM Management is feature in REX whereby the internal ROMs may be replaced by specific blocks in REX. There are 4 blocks reserved for this - block 2 for the "Primary" Main ROM, and block 4 for the "Secondary". In the M100/T102, only 32k is required for the Main ROM, but in the T200, 64k is needed (occupying block 3 and 5 also).
The Primary main ROM is the power up default for REX, and should be handled carefully.
However, REX is not able to deduce if the original main ROM is in use, or not. So, we have to help the system out by following a simple rule.
Any ROM image that is installed in REX for use in Main ROM Replacement must have a small signature included.
This signature is the chaning of the word "MENU" to the word "Menu" at offset 0003h in the standard binary. When REX sees lower case characters, it "deduces" that the original internal ROM is no longer in use, and one of the REX based main ROM images is in use.