汇编：段寄存器和偏移寄存器

Question

I'm having a little trouble understanding where the 10H value is coming from? The example says that ds=1275H (?segment reg.) And bx:457H (?offset) why is ds being multiplied by 10H. And do they mean 1275 is the hex value(so dec. value is 4725)?

Answer 1

The segment register points to a 16 byte block of memory...

This is where the confusion starts. The segment register points to a 65536 byte block of memory and the offset register tells how many bytes to add to the start of this 65536 byte block. Adding a word offset to the start of a 16 byte block would make no sense!

These 64KB blocks are heavily overlapped in memory. Their start addresses only differ by a mere 16 bytes! You could look at these 64KB blocks as forming an array and what the segment register contains then is a zero based index to its elements.

It's thus the spacing between these blocks that is the true meaning of the 10H that you're asking about.

To get to the linear (real) address of the memory location, we need to multiply the value in the segment register with 16 (this is represented as 10h in hexadecimal notation) and then add the offset to it.

Answer 2

The key words here are "the segment register points to a 16 byte block of memory". If you arrange the whole memory in 16-byte blocks, a segment register (for example DS) would contain the number of the block.

To get from the value of the segment register to the linear address, you have to multiply by 16, which is the same as hex number 10, which is usually written as 10H.

After calculating the segment's starting address, add the offset register.

Yes, 1275H is a hex number, that's what the "H" denotes. It corresponds to 4725 decimal.