Mainframe Main Memory access

 Main memory is divided into block. Each block is associate with a storage key. To fetch or update data in the block, program access key is checked against the block key. This is for compatibility with old machine architecture before virtual storage is available. Also the block has a reference and a change bit. 

Main storage blocks are 2K in size though most manipulation is 4K. The he reason is to be compatible with DOS. 

MCU control the access to memory by COU or channel. Address is stored in memory address register (MAR) and data to store or the result of fetch is stored in memory data register (MDR). 

To store, CPU (or channel) alerts MCU with a MEMORIZE signal to signify it is a store request. It then update the MAR and MDR. It then issues the NOW signal to start the store operation. 

To fetch from memory, the request or issue a RECALL signal, then update the MAR. It issues a NOW signal to MCU to fetch the data