the point a to the ground level let us consider an example wherein the cpu 21 includes an address space of 16 binary digits such that an address may be expressed as a e0 a e15 a e0 representing the lowest digit and a e15 representing the highest digit if the address is written in hexadecimal for convenience the address space is 0000 ffff let us assume now that the cpu 21 assigns adresses 6000 dfff to the dram 24 this means that the cpu 21 writes in and reads from the dram 24 by specifying addresses in the range of 6000 dfff and that the other addresses 0000 5fff and e000 ffff are used for an access to a program storing read only memory rom or a refresh of the dram 24 in this situation the highest three digits a e13 a e14 and a e15 of the address alone determine as shown in table 1 whether the system is in the read write mode of operation or in the refresh mode of operation table 1______________________________________ corresponding hexadecimal sup a e15 sup a e14 sup a e13 mode address______________________________________l l l refresh 0000 1fffl l h 34 2000 3fffl h l 34 4000 5fffl h h read write 6000 7fffh l l 34 8000 9fffh l h 34 a000 bfffh h.