LC-3: BLKW ho to specify memory location to store data at? - memory

Question

In LC-3 when you use BLKW how do you initialize the block of memory to be at location x3102 instead of the next available memory location?

Answer 1

First, let's make a side note that all the memory of the 16-bit address space is there for you to use, according to the memory map (e.g. at least in the range x3000-xFDFF), and, it is initialized to zero; you can use it at will.
Generally speaking, the LC-3 assemblers don't allow multiple .ORIG directives throughout the file, instead, they require one at the beginning of the file. If they did allow subsequent .ORIG directives, this would be a way to accomplish what you're asking about.
But even if they did, frequently, we'd run into instruction offset encoding limitations. So there's an alternate solution I'll show below.
But first, let's look at the instruction offset/immediate encoding limitations.
The usual data memory access instruction formats have a very limited offset, only 9 bits worth (+/- about 256), and the offset is pc-relative. So, for example, the following won't work:
.ORIG x3000
LEA R0, TESTA
LD R1, TESTA
LEA R2, TESTB ; will get an error due to instruction offset encoding limitation
LD R3, TESTB ; will get an error due to instruction offset encoding limitation
HALT
TESTA
.FILL #1234
.BLKW xFA ; exactly enough padding to relocate TESTB to x3100
TESTB
.FILL #4321 ; which we can initialize with a non-zero value
.END
This illustrative: while this will successfully place TESTB at x3100, it cannot be reached by the either the LEA or the LD instructions due to the limited 9-bit pc-relative displacement.
(There is also the other practical limitation that as instructions are added the .BLKW operand has to shrink in size, which is clearly painful — this aspect would have been eliminated by supporting a .ORIG directive within.)
So, the alternative for large blocks and other such is to resort to using zero-initialized memory, and referencing this other memory using pointer variables nearby: using LD to load an address, rather than LEA, and LDI to access a value rather than LD.
.ORIG x3000
LEA R0, TESTA
LD R1, TESTA
LD R2, TESTBRef ; will put x3100 into R3
LDI R3, TESTBRef ; will access the memory value at address x3100..
HALT
TESTA
.FILL #1234
TESTBRef ; a nearby data pointer, that points using the full 16-bits
.FILL x3100
.END
In the latter, above, there is no declaration to reserve storage at x3100, nor can we initialize that storage at x3100 with non-zero initialization (e.g. no strings, no pre-defined arrays).
A data-to-data pointer, TESTBRef is used. Unlike code-to-code/data references (i.e. instructions referencing code or data), data-to-code/data
(i.e. data referencing code or data) pointers have all 16-bits available for pointing.
So, once we use this approach, of simply using other memory, we forgo the automatic placement of labels after other labels (for those other areas), and also forgo non-zero initialization.
Some LC-3 assemblers will allow multiple files, and these each allow their own .ORIG directive — so by using multiple files, we can place code&data at varied locations in the address space. However, the code-to-data instruction offset encoding limits still apply, and so, you'll likely end up managing other such memory areas manually anyway, and using data pointers as well.
Note that the JSR instruction has an 11-bit offset so code-to-code references can reach farther than code-to-data references.