On Wed, 21 Feb 2007 10:48:10 -0800, Charles Mills wrote: >I'm trying to port an S/390 assembler routine to Visual Basic. VB has no >shift operators. I'm trying to simulate SRL R6,8 > > > >No need to know Visual Basic, just pretend the following is pseudo-code. R6 >is defined as a signed, 32-bit (31-bit in 390-speak) integer. The VB seems >to work correctly; I'm not asking for VB help. I'd just like any of the >mathematicians on the list to tell me if my logic, especially for negative >values, is correct. I'm worried that I may have missed some boundary >condition. (This code could be shortened to one line, but I did it this way >to make the logic and the debugging clearer.) > > > > ' Simulate a shift right logical 8 > > If R6 >= 0 Then > > R6 = R6 \ 256 ' \ is integer division > > Else > > R6 = R6 \ 256 ' \ is integer division > > R6 = R6 - 1 > > R6 = R6 And &HFFFFFF ' Equivalent to S/390 X'FFFFFF' > > End If >
I don't think you want the "R6 = R6 - 1". And if you drop it, you may as well rethink whether you want the if/else at all, and just do the "And" whether it's needed or not. I'm not a mathematician, nor do I know Visual Basic, but the result produced by this VBS script may be relevant. R6 = -512 R6 = R6 \ 256 And &Hffffff Wscript.echo R6 I'm pretty sure this script uses signed 32-bit integers, because it won't run if i try to set R6 to a value > 2147483647. I picked -512 (hex fffffe00) as a test value, expecting a result of hex 00fffffe which is decimal 16777214, and 16777214 is indeed the result displayed by "cscript srl.vbs" from the command prompt. Bill ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
