Introduction To Computer Science 38
Memory The computer has a memory that can hold both data and also the program processing that
data. In modern computers this memory is known as RAM(Random Access Memory). The
following are items that characterize the Memory (RAM) in the Von Neumann architecture:
– Consists of many memory cells (storage units) of a fixed size.
– Each cell has an address associated with it: 0, 1, …
– All accesses to memory are to a specified address.
– A cell is the minimum unit of access (fetch/store a complete cell).
– The time it takes to fetch/store a cell is the same for all cells.
– When the computer is running , both program and data also known as
variables are stored in the memory.
– Typical memory size in a personal computer (PC) is 4GB- 8GB.
– Typical memory access time or speed in a personal computer is in the
range of nanoseconds.
– It is volatile in that it can only store when power is on
– Typical operations in memory are:
√ Fetch(address)- fetching a copy of the content of memory cell with the specified
address.
√ Store(address, value)- storing the specified value into the memory cell specified
by address.
– The memory is interfaced via:
• Memory Address Register,
• Memory Data Register,
• Fetch/Store signal.
• Thus, the two memory operation fetch and store amounts to:
– Fetch(address)
– Load address into MAR.
– Decode the address into MAR.
– Copy the content of Memory cell with specified address into MDR.
• Store(address, value)
• Load the address into MAR.
• Load the value into MDR.
• Decode the address in MAR.
• Copy the content of MDR into memory cell with the specified address.
