Mainframe floating point math implementation.

On Mar 13, 2016, at 4:39 PM, Charles Anthony <charles.unix.pro at gmail.com> wrote: The Multics distribution includes ISOLTS, a surprisingly complete and pedantic processor test program. It is unhappy with our emulated floating point. This should be the floating point used by the GE 6xx series and the Honeywell DPS8 and 6000 series. There is one particular failure that I am driven to seeking help for. If the intricacies of mainframe floating point math h/w do not interest you, time to delete this message and move on. For add and subtract operations, the operand with the smaller has its mantissa shifted right and the exponent incremented adjusted until the exponents match. From the DPS8M assembly language manual: "The mantissas are aligned by shifting the mantissa of the operand having the algebraically smaller exponent to the right the number of places equal to the absolute value of the difference in the two exponents. Bits shifted beyond the bit position equivalent to AQ71 are lost." Sadly, ISOLTS complains about our implementation. It does helpfully provide what it says are the correct answers. Examination of the answers reveals the it is not the case that the shifted bits are lost; the shift mantissas are rounded according to rules that I can't quite characterize.