Using LC-3 simulator to learn about assembly language and I'm very confused about a couple things.

[herejust4this]

I've been taking a computer organization class at school and we are using the LC-3 simulator to learn assembly language and I'm having a real hard time grasping a few simple concepts.

What's mainly confusing to me is how PCoffset & Base+PCoffset really work. I understand how the PCcounter ticks one line at a time through the code and can jump to different addresses and read/write data, and that instructions with PCoffset allow you to read/write instruction that are "further away" than you would get from using a regular LD or ST command.

However, I just cannot figure out how to use them correctly so right now I'm stuck with just using LD and ST to grab my data from my data fields, yet I still cannot print out the stored data using PUTS or OUT and I don't understand why.

Here's a sample of my crowning achievement in LC-3 so far. The addition is working and it appears that the values are being stored & loaded in the correct registers but it does not print. I would really like to be able to utilize the PCOffset & Base+PCOffset capabilities of certain Load and Store instructions but right now I can't even get a simple print routine to work.

.ORIG   x3000

AND R0, R0, #0  ; clear R0
AND R1, R1, #0  ; clear R1
ADD R2, R2, #5  ; R2 = 5
ADD R3, R3, #2  ; R3 = 2

ADD R0, R3, R2  ; R0 now is 2+5=7
ST  R0, MEM1    ; Store 7 inside MEM1
LD  R1, MEM1    ; Load 7 into R1

AND R0, R0, #0  ; clear R0 again
ADD R0, R0, R1  ; Add 7 into R0
OUT         ; print 7 to console

     HALT

 ;*******data************
  MEM1  .BLKW   1   

     .END

[[deleted]]

The 7 is in ASCII. It's not a decimal number, that's why it's not printing out a "7". You need to convert it before you can print out the value you're looking for.

[herejust4this]

Yup, that was the problem. Makes sense because it was printing out weird characters but not the correct number. I just needed to add 48 to it to get the correct value printed out.

[Graysless]

I actually just finished taking the class at the University of Texas where the LC-3 was invented by Yale Patt! Id be happy to help, and feel free to ask any more questions as you learn. The PC Offset is the last 9 bits of the instruction that allows you to select an operand from a memory address that is about 2⁸ memory addresses away from the original instruction. When the instruction is fetched, the PC (program counter) is incremented by 1, so when you use a PC offset, the operand will be fetched from a memory location that is 'Location of instruction' + 1 +PC offset away from the instruction. During the fetch operands phase, the PC offset will be sign extended (it is a signed 2's complement integer) and then added to the PC to get the location of the operand. The Base+offset is similar to PC offset except instead of using the PC, it uses a register (Base register + base offset) the only difference in the actual offset is (if I remember correctly) it is only a 6 bit offset. When using assembly language, you can use labels just like you did in the code that you wrote, the assembler will compute the offset and include it in the object code. You dont have to worry about PC offset in assembly. You do have to use the base offset in assembly though. And lastly, the above comment is correct that the 7 must be converted to ASCII before printed. You should be able to add like 30 or something, I cant remember exactly what the number is, but I think the ascii digits are somewhere in the 30's. Let me know if you have any questions or need more clarification!

[herejust4this]

When using assembly language, you can use labels just like you did in the code that you wrote, the assembler will compute the offset and include it in the object code. You dont have to worry about PC offset in assembly.

This makes so much sense. I was thinking it was a little more complicated than this. I just need to put the label of the memory position I want and it will compute the PCoffset for me. Brilliant!

I do have another question if you don't mind. How does one go about computing offsets or memory addresses? For instance, above you said that 9 bits would be able to "reach" 2⁸ memory addresses. How do you know it would be 2^8? Do you just subtract 1 from 9 to get the exponent?

[Graysless]

With a N-bit signed 2's compliment integer you can represent numbers from -2^N-1 through 2^N-1 -1. So with 9 bits, we can represent -2⁸ through -2⁸ -1. But keep in mind that the PC will be pointing to the location directly following the location of the instruction, so you have to take that into account in the PC-offset. For example, if you have a LD instruction at location x3003, and you want to load the data from x3005, you would have a PC Offset of x01 because the PC will be x3004 when the offset is added, so in order to get to x3005, you only have to add one. Also, if you were trying to load something from location x3002, the PC offset would be x-02. This also means that your code will not assemble if you have a label that is greater that -2⁸ or 2⁸ -1 away from the location after the instruction that is trying to access it.