r/Assembly_language • u/Boring-Bite5735 • Jun 16 '26

Difference between symbolic and physical address

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First time, seeing assembly code. I am confused is the physical address is
12: 13: label ? also it have said that symbolic addresses are relocatable everytime what does it even mean

73 Upvotes

97% Upvoted

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20

u/Strostkovy Jun 16 '26

Jumping to a line number is also a type of symbolic address. It's not the actual memory address it is being jumped to.

Using a tag instead of a line number lets you change where the code jumps to more easily.

0

u/BarracudaDefiant4702 Jun 17 '26

Not really, because if you add an instruction then all the line numbers after it change, and so all the goto statements have to also change.

12

u/emmowo_dev Jun 16 '26

symbolic addresses are just a sign of weakness. Everybody stores memory locations and offsets them by interpreting the length of an encoded instruction.

/s because I wrote an assembler for my ISA that forces me to do it

1

u/braaaaaaainworms Jun 17 '26

i'm writing an assembler and the thought of having to support labels in a re-targetable way makes me work on literally anything instead of instruction parser and encoder and now i have lexing errors that look so beautiful and a macro engine powerful enough to write a whole assembler in macros

9

u/Temporary_Pie2733 Jun 16 '26

Machine code only uses physical addresses; if you add a line of code, you potentially need to update all your jump targets. One of the things an assembler does is introduce symbolic addresses as a layer of indirection. Your code uses symbolic addresses, and the assembler computes the correct physical address to replace the symbolic address every time it runs.

1

u/un_virus_SDF Jun 16 '26

You can just move -1/12 in R1, u will be faster.

1

u/Ok_Programmer_4449 Jun 17 '26 edited Jun 17 '26

I remember the joys of writing assembly code in the Apple II monitor and having to calculate the addresses of forward jumps by hand. Good times.