Why we should reserve a global register in MMIX?

Question

What does "GREG @" in this code do? I know it reserves a global register, but why do we need it here?

            LOC Data_Segment
            GREG @ 
    Y       OCTA 0

    x       IS $1
    buf1    IS $2
    buf2    IS $3

            LOC #100

    Main    FLOT    buf1,157
            FLOT    buf2,50
            FDIV    x,buf1,buf2
            STO     x,Y
            TRAP    0,Halt,0

Answer 1

The MMIX instruction set does not have an absolute addressing mode. Instead, as is typical for RISC architectures, the only available addressing modes are a indexed addressing modes with a base register and either an 8 bit immediate or a register index. Hence, loading variables from memory requires you to first load a nearby address into a register. This is achieved with the GREG @ directive: it allocates a global register with the current address, permitting access to nearby global variables (in this case, that is the variable Y).

In more complex programs, you might probably want to chose a different approach as you'll run out of global registers quickly. One solution is to store a pool of addresses next to your code and load the address of that pool with a GETA instruction like this:

        ...
        GETA    $4, pool     @ obtain the address of the pool
        LDOU    $5, $4, 0    @ load the address of Y from the pool
        STO     x, $5, 0     @ store x to Y
        ...

pool    OCTA    Y            @ literal pool holding Y