On Thu, 8 Nov 2007 10:14:42 -0000, Van Dalsen, Herbie wrote: >Apologies, I keep on forgetting that the '8' just signals the above the >line, you and Tom and all the others are quite correct with the >x'7fffffff', I have it now... it is a pitty that IBM did not use the >lowest bit to signal the line... x'00000001' for 24-bit and x'00000002' >for 32-bit, it would have meant less wastage. Because you would have >lost the first few addresses...
huh? What "wastage" are you talking about? I'd suggest that you read chapter 3 of the principles of operation. What do you mean about "signaling the line"? Are you referring to the bit in the PSW that tells the processor whether to operate in 24-bit mode or 31-bit mode? In 24-bit mode, addresses can go from 0 to x'FFFFFF'. In 31-bit mode, addresses can go from 0 to x'7FFFFFFF'. The line is simply a way of talking about storage locations that cannot be referenced in 24-bit mode In the 360 and 370 architecture (except for the 360-67), addresses were 24 bits, with a maximum possible value of FFFFFF. Because of wrap around, the bext byte after x'FFFFFF' was location 0. The 370-XA architecture allowed for 31 bit addresses. In 31-bit mode, the next byte after location x'FFFFFF' is x'1000000'. Note that it takes a minimum of 25 bits to represent that address. -- Tom Marchant ---------------------------------------------------------------------- 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
