HP 33s - Manual (Page 202)

13–18 Programming Techniques
Loops with Counters (DSE, ISG)
When you want to execute a loop a specific number of times, use the { l
(increment; skip if greater than) or | m (decrement; skip if less than or equal
to ) conditional function keys. Each time a loop function is executed in a program, it
automatically decrements or increments a counter value stored in a variable. It
compares the current counter value to a final counter value, then continues or exits
the loop depending on the result.
For a count–down loop, use | m variable
For a count–up loop, use { l variable
These functions accomplish the same thing as a FOR–NEXT loop in BASIC:
 variable = initial–value ! final–value ! increment
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.
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%! variable
A DSE instruction is like a FOR–NEXT loop with a negative increment.
After pressing a shifted key for ISG or DSE ( { l or | m ), you will be
prompted for a variable that will contain the loop–control number (described
below).
The Loop–Control Number
The specified variable should contain a loop–control number ±ccccccc.fffii, where:
„ ±ccccccc is the current counter value (1 to 12 digits). This value changes with
loop execution.
„ fff is the final counter value (must be three digits). This value does not change
as the loop runs.
„ ii is the interval for incrementing and decrementing (must be two digits or
unspecified). This value does not change. An unspecified value for ii is
assumed to be 01 (increment/decrement by 1).
Given the loop–control number ccccccc.fffii, DSE decrements ccccccc to
ccccccc — ii
, compares the new ccccccc with fff, and makes program execution
skip the next program line if this ccccccc ≤ fff.