Computer Memory

Abstract:

A physical device that is used to store program code or data of a processor on either a temporary or a permanent basis, typically classified into two types:

• Volatile Memory
  • Faster and less costly
  • Used for temporary storage like caching and internal memory
  • • RAM
• Non-Volatile Memory
  • No power is required to hold the data in memory
  • Slower access speed, and more costly
  • Used for long term storage

Most memory can dynamically and flexibly defined by the user, except some fixed memory addresses, such as the internal private peripheral bus.

General:

Memory is accessed by presenting the computer with an address from where data is read or written into. A drawn example of a 8 bit wide memory structure that holds 8bits of data is as follows:

Memory addresses tend to start from the bottom up An address bus of 32 bits can be used to access $2^{32}$ locations of memory, but since each piece of memory for us stores a byte, we can say that it can be used to access $2^{32}$ bytes of memory

Memory Access in Depth:

The two following operations are as follows:

Write Data:

• Provide an address to the memory address bus
• A particular “word” is then selected by the address decoder and connected to our Bit Line Amplifier
  • This may include breaking the memory down into columns and rows for larger memory
• The data gets presented to the Data_In ports of the microprocessor
• The bit line amplifier sets the bit lines to the desired values and then drives that data from itself to the memory cells.

Write Data:

• Provide an address to the memory address bus
• A particular “word” is then selected by the address decoder and connected to our Bit Line Amplifier
  • This may include breaking the memory down into columns and rows for larger memory
• The selected data is connected to the bit line amplifier;
• The amplifier restores the signal to the proper voltage level, then outputs it to the Data_Out ports of the processor;

For an explanation of every section of the Memory Map: Here

Program Image:

You can think of this almost as a start up routine for the microcontroller, It contains:

• Vector table
  • Includes the starting addresses of exceptions (vectors) and the value of the main stack point (MSP);
• C start-up routine;
• Program code
  • Application code and data;
• C library code
  • Program codes for C library functions.

Here’s the program image for the ARM Cortex M0: 1st Instruction here is: Branching to the starting of the programme execution address (reset handler) 2nd Instruction is executing the program.

Memory Use By Program: