Section 11: Branching and Looping164
The cc portion of the number tells the calculator how you wish to count. Current counter value iiiii is incremented or decremented by the increment value of cc. If you do not specify a cc value, the HP-41C assumes you wish to count by ones (cc=01). A cc value must be specified as two digits (e.g., 01, 03, 55).
Increment and Skip if Greater
Each time ISG is executed, it first increments iiiii by cc. It then tests to see if iiiii is greater than fff. If it is then the HP-41C skips the next line in the program.
So, if you stored the number 100.20001 in storage register R10, the ISG 10 instruction would begin counting at 100, would count up until the counter was greater than 200, and it would increment by 1 each time the loop was executed.
  Contents of storage register R10 = 100.20001
  Execution of ISG 10 would:
    Start counting at 100.
    Increment by 1.
    Test to see if counter is greater than 200.
After one execution or pass through the loop, R10 would be 101.20001. After 10 executions or passes through the loop, R10 would be 110.20001. Each time ISG 10 is executed it checks to see if the counter is greater than 200. When it is greater than 200, it skips the next line of the program. You will see how skipping the next line in the program is useful in a moment.
If you execute ISG from the keyboard, it simply increments the specified register just like it would in a program, but no program lines are executed or skipped.
Decrement and Skip if Equal
Each time DSE is executed, it first decrements iiiii by cc. It then tests to see if iiiii is equal to (or less than) fff. If it is then the HP-41C skips the next line in program memory.
So, if you stored the number 100.01001 in storage register R11, the DSE instruction would begin counting at 100, would count down until the counter was equal to (or less than) 10, and it would decrement by 1 each time the loop was executed.
  Contents of storage register R11 = 100.01001
  Execution of DSE 11 would:
    Start at 100.
    Decrement by 1.
    Test to see if counter is equal to (or less than) 10.
Remember, in a program when the final vlaue is obtained, the HP-41C skips the next line in the program. You will see how this is useful later.