HACK Machines and its assembler

Question

couldn't really get much information about the virtual register. All i know is the R0 to R15 which is address 0 to 15 are pre-defined.

Is virtual register = virtual machine? so what is a virtual register in the context of Hack machine and its assembler?

Answer 1

I've have never heard of HACK machine before, so I looked for it on Google and found the official page, read a couple of PDFs and make a simple test. All this just to say: I have 5 min preparation on this subject :)

Virtual Register are just symbols, names for numbers. R2 is just another way to write 2.

The manual states

To simplify assembly programming, the symbols R0 to R15 are predefined to refer to RAM addresses 0 to 15, respectively.

I initially thought you could use them like this R3=D-1 or even R3=R2+R3. In the HACK machine accessing memory is very verbose as you can only address it indirectly by the use of the A register, so I thought that Virtual Registers could be used to automatically generate more complex code, say by assembling R3=R1+R2 into

@1 D=M @2 D=D+M @3 M=D

But this is not the case, you can use them only with the @ instruction and I don't see how this would simplify assembly.

They can make it more readable if you stick to the convention that memory locations are loaded into A only by using labels, so that if you see a @R10 you known/expect that A is loaded with an address and when you see an (equivalent) @10 you know/expect that A contains a values for arithmetic computation (say adding 10 to D).
This is just pure semantic however, a matter of how you use the assembler, like naming string variable with str prefix, and have not meaning to the assembler itself.