Answer: Weekly Classic Computer Trivia Question (20150112)

More discussion ensued than was expected for this question. On a minimal 4k machine (no extended memory and no EAE) and no data break devices operating the longest latency would be that of a memory reference instruction with a defer (indirect address) cycle. On a Straight 8 this would be 4.5 microseconds. A single cycle data break device could have delayed the interrupt indefinitely although I know of no such devices. A three cycle break could extend the delay by an additional three memory cycles (4.5 microseconds) for a total of 9 microseconds. If the machine was equipped with an EAE then the Division instruction would have delayed entry of the interrupt service routine by up to 35 microseconds (on a straight 8). I believe this was a 9 microsecond instruction on the 8/e EAE. If the machine is equipped with extended memory then there is the CIF induced delay. CIF is the Change Instruction Field instruction. This instruction only schedules the change. The actual change does not take place until the next JMP I or JMS I instruction. The interrupts are masked by the CIF until the conclusion of the indirect branch. This is because the interrupt handler would not be able to restore the instruction field correctly in this instance. I have been unable to think of a reason why you would ever want to delay the indirect branch although you certainly could do so. That means the sequence CIF followed by a JMS I would only see the additional delay from the JMS I which is the same as the delay from a memory reference instruction with a defer. The JMP I variant would only be 3 microseconds. And the greatest source of interrupt latency on a straight 8 would have been due to the Option 189 which is the low cost A/D converter. This option added the ability to do 6 to 12 bit A/D conversions as a CPU instruction (6004). It performed a typical successive approximation conversion using the MB and AC registers. When 12 bit conversion were selected this instruction would take 56.6 microseconds to execute according to the manual. I don't have a 189 for my straight 8 but I am planning on hunting down the cards I don't have. And the answer Mr Stearns gave for this question was "No matter what it is faster than Windows." On Mon, Jan 12, 2015 at 9:10 AM, Doug Ingraham <dpi at dustyoldcomputers.com> wrote: