CODEWITHSUNDEEP
assemblycpu-registerslc3immediate-operand
user1874239
user1874239

Reputation: 325

Load a value into a register in LC3

Is there a single LC3 instruction to load a value into a register? I need to write some assembly code and I come to where I need to write just a single instruction to store the value 1 in R1.

Upvotes: 2

Views: 12025

Answers (3)

Peter Cordes
Peter Cordes

Reputation: 364190

LC-3 doesn't have a move-immediate. The instruction set includes AND/ADD/NOT (with register or immediate), LEA, and load instructions.

Using add-immediate with a zeroed register

If you have a zeroed register, you can materialize any small constant into a register with add (5-bit sign-extended immediate, value-range [-16, +15]).

(Zero is special: It can be created by AND regardless of previous register contents.)

 and  r7, r7, #0        ; R7 = 0.  Somewhere in your program or function

 add  r0, r7, #-16      ; R0 = 0 + -16 = -16
 add  r1, r7, #15       ; R1 = 15

The source register doesn't have to be zero; any other known constant within -16..+15 of your desired value can work. e.g. if you know R4 is 3, you can get R1=1 with add r1, r4, #-3 + 1. But that's complicated to keep track of, probably only worth doing if you're optimizing for size or speed.

Load a value from static storage

ld uses a PC-relative addressing mode so it can load nearby words (-256 .. +255).

  ld r1, ONE


ONE .fill 1        ; elsewhere in your program

If you need a couple simple constants at various different places in your program, and don't want to leave one of your precious few registers zeroed all the time, this is a single-instruction way to get a constant that doesn't require any setup.

LEA to create a non-zero value

If you want to just return 0 or non-0 from a function, you may not care which non-zero value you create. and can only clear bits, not set them. add and not aren't safe either if you don't know the value of any register; there's always an input that will result in 0 for them. e.g. add r0, r1, #1 wraps to 0 if the original value was 0xFFFF aka -1.

But LEA can create a value that depends only on the program counter (which is known) and the 9-bit signed immediate in the instruction ([-256 .. +255] range). We know that address 0 isn't going to be in the middle of our program, so any address is fine if we just want non-0. Often we begin LC-3 programs at .orig x3000, so small numbers like 1 can't be generated by LEA, but if we just need non-0 that's fine.

.orig x3000


...
 lea r0, nonzero
...


nonzero       ; put this label literally anywhere in your program
var1   .fill 1

The nonzero label is attached to the same address as var1, which is totally fine. You could even put it on the LEA instruction itself, like

nonzero  lea r0, nonzero

If you can use a .orig that puts your program close to the start of memory address-space, small constants like 1 can be created with LEA. Like perhaps

.orig  0
address0

...

  lea r1, address0+1       ; R1 = 1, if this is the right syntax for this

...

Upvotes: 0

TirusTheVirus
TirusTheVirus

Reputation: 11

Yes there is. it is: LD R0,Param1

What this means is that it will load the value of Param1 into Register 0.

Upvotes: 0

Nexion
Nexion

Reputation: 433

LD R1, value

Do a .fill on "value" with whatever value you want to load into R1. I don't believe there's a way to load a literal value into a register with a single line, but you can do it with one instruction and one label.

Upvotes: 2

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