So I\'m confused on the LDR opcode thing which is in assembly language. here\'s

Question

So I'm confused on the LDR opcode thing which is in assembly language. here's an example. LDR R4, R2, #-5 I understand the basic idea. For example, for LD R4 #-5, this would mean, take a look at PC and increment it. After doing so, subtract 5 from it. then go that whatever THAT memory is. And put it into R4, which is the destination register. I understand that where the Base is PC+1 and the offset is whatever you put in it. I mean, I know what PC is. I can just take a look at it wherever the last memory location instruction was executed for example. But I don't understand for LDR. I know what it's saying. It wants me to do R2 (which is base) subtract that by 5. But I mean, WHAT'S R2? Is it 2? What's the value assigned to it? I hope you understand my question. Basically I'm confused on what R2 stands form. I know the idea where you gotta subtract 5 from R2. Then go to that memory location and store it in destination R4. But basically, WHAT IS R2 and it's corresponding value? Thanks and I will give a lifesaver to whoever answers it to the point where I understand what they mean. Muchas gracias! If you know, ANSWER FAST! I need this so I can finish my program. Thanks!

Explanation / Answer

LDR syntax is like this:

LDR DestinationResister, BaseResister,Offset

But the syntax of LD is like this:

LD DestinationResister,Offset

In LD the base resister is always PC. But in LDR you can choose any resister as base resister.

In your example

LDR R4, R2, #-5 ----> you have taken R2 as base register, suposse you have stored 1000H in R2.

So by executing the above instruction the value at memory 1000-5=(0FFB)H , will be loaded into R4.

Here H represents the Hexadecimal number.

Similarly you can take any resister as you wish.

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