Military Time 2899

John D. Slayton

360s had a 32bit, binary timer ... located at location 80 (hex '50') in real storage. it was about 15hr period ... and most machines updated it about 3milliseconds. some machines had high-performance timer option which updated the low bit approx. every 13microseconds.

conversions to various other time representations were handled by software.

370 introduced 64bit hardware clock ... hardware spec called for machines to update the timer on approx. same period as instruction end time ... but as if bit 51 represented one microsecond (bit 63 is 1-4096s of a microsecond) ... which made bit 31 equal to 1024-1000 seconds i.e. top 32 bits is approx. a second timer giving somewhat over 4billion seconds.

standard called for "zero" time to be the first second of the century. it has cycle period of approx. 143 years. again software provides converting from the hardware clock to various other time representations.

the memory location 80 timer was eventually dropped ... part of the issue was the excessive memory bus traffic generated by the constant clock location updating.

detailed discussion of TOD clock

format description of tod clock

setting and inspecting the tod clock

more recently an extended tod clock is defined with 104 bits ... format description

however the extended hardware clock has 8 additional bits defined as zero prefixing the tod value and 16bits postfixing the value (128bits total)