HP HP-67

238 Using the I-Register for Indirect Control
Indirect Incrementing and
Decrementing of Storage Registers
In section 11, you learned how to increment or decrement the
I-register by using the instructions (5Z) and . By using the number
in the I-register as an address, the instructions 3 and B
increment or decrement the contents of the storage register
addressed by the number in I.
The indirect addressing of the storage registers for and is
the same as that for £ ([, ([, and storage register arithmetic
using (1. When using and (O5Z0), the calculator looks at
only the integer portion of the absolute value of the number stored
in the I-register. An attempted or operation when the
number in I is 26 or greater results in an error condition.
and function very similarly to (5Z) and (052). When an
or instruction is performed in a running program, the
calculator first increments (adds 1 to) or decrements (subtracts 1 from)
the contents of the storage register addressed by the number in the
I-register. If the contents of the storage register addressed by the
number in I are then zero (actually, if they are between —1 and +1),
the calculator skips one step. If the contents of the storage register
addressed are not then zero, execution continues with the next step
of program memory after the or instruction.
Indirect Control of
Branches and Subroutines
Like display control using B3 [fi] and addressing of storage registers
using @ and [@, you can address routines, subroutines,
even entire programs, with the I-register.
To address a routine using the I-register, use the instruction @ .
When a running program encounters a (&) [fi} instruction, execution
is transferred sequentially downward to the that is addressed by
the number in the I-register. Thus, with the number 7 stored in I,
when the instruction [@ is encountered, execution is transferred
downward in program memory to the next 7 instruction before
resuming.