; Define memory locations for user input buffer and opcode output
INPUT_BUFFER .WORD x2000 ; Memory location to store user input
OPCODE_OUTPUT .WORD x2004 ; Memory location to store opcode output
I tried = . and =. they didn't work unfortunately
You're looking for .FILL ... to provide a value for an initialized data word as .word ... would work in other assembly languages.
INPUT_BUFFER .FILL x2000
OPCODE_OUTPUT .FILL x2004
Use .BLKW ... to provide a count of a number of zero initialized data words.
There is no equ or equate or #define or = to declare numeric constants as far as I know.
But .word doesn't declare a constant either, it declares an initialized data word, which is exactly what .fill does. (There is a difference in that that .word ... usually accepts multiple comma separated values, whereas LC-3's .fill ... requires one argument per line.)
You can use these labels with LC-3's indirect loads and stores, LDI and STI. For example,
LDI R0, INPUT_BUFFER
will use the data memory location at INPUT_BUFFER to get the address of where to go to get the value to put into R0. So, the instruction itself refers to INPUT_BUFFER, and INPUT_BUFFER stores x2000, which is the effective address used to fetch from memory for R0.
You can also load the pointer value directly:
LD R1, INPUT_BUFFER # now have x2000 in R1
LDR R0, R1, #0 # load from location at R1+#0, so x2000
And further, you can increment/modify the memory location at INPUT_BUFFER, so
LD R1, INPUT_BUFFER
ADD R1, R1, #1
ST R1, INPUT_BUFFER
That will change the memory location at INPUT_BUFFER to x2001 (assuming it was x2000 to start with), so for example, a next LDI or STI to that memory location will ultimately address memory location x2001.
Personally for small algorithms I would prefer to load pointers into registers and use them there including incrementing in registers in loops for example. LC-3, with 8 usable registers, is register rich compared to MARIE, HACK, LMC and some others.