Abstract: An addressing device is described for a control store comprised of a read-only storage portion and a read-write storage portion, each divided in addressable microprogram sectors and both accessible through a common address register and accessing to a common read-out register. Said control store is a part of a multimicroprogrammed unit further comprising a logical unit, a status register for said logical unit and an automatic store address forming organization operating on data supplied from the common read-out register and the logical unit. When a microprogram stored in a sector of the read-write storage portion must be substituted for a microprogram stored in a sector of the read-only storage portion, the logical unit forces into the status register, a pointer code and the sector addresses of the concerned read-only and read-write storage portions.