John McKown writes: <begin extract> No such thing as a negative displacement. A displacement is more like an unsigned immediate operand. From 0..4095 (0x000 to 0xFFF for a 12 bit displacement) or 0..1048575 (0x00000 to 0xFFFFF for a 20 bit long displacement) </end extract>
This is is correct only for traditional System/360 instructions. As one of many counter-examples consider the GRASL instruction, about which the PrOp says: <begin extract> The contents of the I[2] field are a signed binary integer specifying the number of halfwords that is (sic) added to the address of the instruction to generate the branch address. </end extract> In general the 12- and 20-bit "displacements" in the new instructions are signed. Le bon Dieu est dans le détail. --Gustave Flaubert John Gilmore, Ashland, MA 01721 - USA
