Why we should reserve a global register in MMIX?

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

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