s5 is used to record “memory full”. Here’s an example:<\/p>\n
\r\n ; A= 02009999999999 \r\n05552 1 -> s5 ; S= .....5.....b\r\n05553 memory full -> a ; A= 02009999999990\r\n05554 0 -> p\r\n05555 if a[p] >= 1\r\n05556 goto 05560\r\n05557 0 -> s5 ; S= ...........b\r\n05560 0 -> a[w] ; A= 00000000000000\r\n<\/pre>\nInitially, program memory starts with instructions for the [A] to [E] keys and then is padded out with NOPs. You see these as key code “35 01″ if you SST far enough in W\/PRGM mode.<\/p>\n
The ‘memory full -> a” microinstruction checks the end of program memory. If it contains a 0 (the program code for a NOP), A[0] gets set to zero. If not, you get something non-zero.<\/p>\n
The “0 -> p” and “if a[p] >= 1” check this. If A[0]=0 step 05557 happens and s5 “memory full” gets cleared. If A[0] isn’t 0, that step is skipped over and s5 stays set.<\/p>\n
Interestingly, if the program pointer (code 075) is in step 101, ie current step is step 100, this also results in A[0] being non-zero. This effect is used to show the “at step 100” leading and trailing minus signs.<\/p>\n
NOTE: s5 only has the “memory full” meaning when you are in W\/PRGM mode. In run mode, the same flag is used for error<\/a>. It never confuses the two because, unlike modern interrupt-driven systems, the microcode polls the RUN-W\/PRGM switch. This allows it to switch contexts when it is safe to do so (after the current activity has completed).<\/p>\n