The HP-65 calculator has an internal set of status flags known as the “s register”. “s3” is one of those flags but it behaves a bit strangely … <\/p>\n
Any check of s3 doesn’t do what you’d expect. For example:<\/p>\n
\r\n ; S= ............\r\n00537 if s3 = 0\r\n00540 goto 00552 ; ??? didn't go 0552 yet s3 doesn't seem set\r\n00541 if s11 = 0\r\n<\/pre>\ns3 is clearly 0 yet “if s3 = 0” didn’t go where one would expect.<\/p>\n
There is something going on “under the hood” here. The tests(num) function that does the “if sN = 0” microcode instruction sets carry if sN is 1; OR rN is 1. More flags!<\/p>\n
rN is labelled in the code as “hard status for HP-55”. It looks like there are some hardware attachments of some type that can be wired in to override individual bits of the s register. This is similar to the s3 flag being used by the HP-67 for status information from the card reader (eg execute a “CRC ready? command” then test “if s3 = 1”). With the ’67 it is obvious because the hardware actually sets the s3 flag. It shows as “S= …3……..”.<\/p>\n
The catch with the ’65 is the s3 flag appears clear but tests as if set (because r3 is). The behaviour doesn’t match what the presented status info suggests.<\/p>\n
The only use of the 12 HP-55 “r” flags in the HP-65 is r[3]. This is 1 if the relevant slide switch is at W\/PRGM and 0 if it is in the RUN position.<\/p>\n
So s3 means W\/PRGM – but you’ll never notice it being set. Something to be aware of in microcode traces.<\/p>\n
It might have been better if s3 were directly tied to the W\/PRGM-RUN switch and couldn’t be cleared or set by software. I’m not sure whether the HP-65 microcode ROMs would work with that but, if so, it would show what’s happening a lot better.<\/p>\n","protected":false},"excerpt":{"rendered":"
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