Interpretation of memory layout

Question

On this page, there is an example image of part of memory layout for a program :

What does each line in such an image represent? Does each line represent a single line of physical memory ? Usually physical memory has 32-bit or 64 bit as each line, so in that case, does each line in the image cover several lines of physical memory ?

Thanks.

Answer 1

Each two digit group represents one octet (byte). They are often laid out in groups of 16 octets because it fits well on a printed page or terminal screen. The address of the leftmost octet is in the left column (e.g. 00430020).

This representation is used as a typographic convention and does not necessarily have any relation to the physical structure of memory.

Answer 2

One would hope it was patently obvious, but...

Each line in the above diagram represents 16 bytes (given that the address advances by hex 0x10 with each line and given that there are 16 bytes on each line).

"Usually physical memory has 32-bit or 64 bit as each line" -- Well, no. Physical memory is primarily divided into 8-bit bytes. The computer may have a 32 or 64-bit wide transfer path from memory to the CPU, but the width of that path is irrelevant to understanding diagrams such as the above. (The term "line" inside a computer basically applies only to "cache line", which is a group of bytes from maybe 16 bytes long to possibly 256 bytes long (depending on the design) which are together in a "cache" -- a high-speed "snapshot" of portions of memory. But such cache operates "transparently" so you can ignore its existence for most purposes.)

What you will observe in the above diagram is that the data in a 32-bit address is "little endian" -- the first "next" field is 30 00 43 00, while the location it's pointing to is 00430030. The bytes in memory are reversed from how you or I would "naturally" read them.

So this diagram is simply showing some simple structures in memory and how they address each other.