The PSW has two bits which, together, indicate the AMODE. They are the EA bit, bit 31. And the BA bit, bit 32. Now, the PoPS clearly states that having the EA bit ON and the BA bit OFF is invalid and causes a "specification exception" to be raised. Now, I don't know how the hardware works. But one reason that I can think of as why the ON/OFF combo is invalid is due to how the hardware tests for the current AMODE. If it first tests the BA bit for '0'b to see if the AMODE is 24, then if would make sense to require the EA bit to be OFF in this case as it would not be tested. So they made it required for completeness. IE, hardware AMODE is determined somewhat like:
IF BA=0 THEN AMODE=24 ELSE IF EA=0 THEN AMODE=31 ELSE AMODE=64 This would test the AMODE in order of their evolution in the hardware. An alternate evaluation could be: IF EA=1 THEN AMODE=64 ELSE IF BA=0 THEN AMODE=24 ELSE AMODE=31 Anyway, strickly for the purposes of speculation, what sort of addressing mode could a future z architecture make up which would use EA=1 BA=0? And how much overhead in the hardware would it cost? it would cost because we don't currently need to test EA if BA is 0 by the first scenario or to test BA if EA is 1 in the second. Or maybe I should just take another pain pill and go to the rest of the way to sleep. John McKown Systems Engineer IV IT Administrative Services Group HealthMarkets(r) 9151 Boulevard 26 * N. Richland Hills * TX 76010 (817) 255-3225 phone * (817)-691-6183 cell john.mck...@healthmarkets.com * www.HealthMarkets.com Confidentiality Notice: This e-mail message may contain confidential or proprietary information. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. HealthMarkets(r) is the brand name for products underwritten and issued by the insurance subsidiaries of HealthMarkets, Inc. -The Chesapeake Life Insurance Company(r), Mid-West National Life Insurance Company of TennesseeSM and The MEGA Life and Health Insurance Company.SM
