The Indirect Register I
Another powerful feature of the HP 15C is indirect addressing. The registers I and (i) can be used not only for loop control (for DSE/ISG) but it can also be used to call different memory registers, call different subroutines, and control flag operations.
We will work with the registers I and (i) in Parts 12 and 13. Part 12 will cover how to use the index registers with data recall and Part 13 will cover how to use the index registers for calling different subroutines.
Index Register and Memory
The HP 15C Manual names the memory register I (RI) the as the Index Register. You can store and recall values in memory register I just like any other memory register. Here is where it gets good: you can use register I to store and recall any other memory register with the integer part of I as the designated register. This is where the variable (i) comes in.
Let I = N where 0 ≤ N ≤ 19* (see Caution below)
STO (i) stores the contents in the display into register N.
RCL (i) recalls the contents of register N.
Caution: The value of N can be anywhere from 0 to 65. The upper limit of N is determined by how the memory registers are partitioned. You can check the number of registers have been partitioned by pressing [ g ] [RCL] (MEM). It is the left-most number. By default the HP 15C partitions 19 as the highest memory register number allowed. For the purposes of this tutorial we will use the default settings.
Example:
Store 2 in RI. Then store π/4 in R2 indirectly (by use of RCL (i)).
Key Strokes:
2 [STO] [TAN] ( I )
[ g ] [EEX] ( π ) 4 [ ÷ ] [STO] [COS] ( (i) )
[backspace] * To clear the display
[RCL] 2 * Display: 0.7854 ≈ π/4
Indirect Addressing - Memory Registers
If RI contains... STO (i) and RCL (i) will address
0 R0
1 R1
2 R2
3 R3
4 R4
5 R5
6 R6
7 R7
8 R8
9 R9
10 R.0
11 R.1
12 R.2
13 R.3
14 R.4
15 R.5
16 R.6
17 R.7
18 R.8
19 R.9
20 to 65** R20 to R65**
On final note, storage and recall arithmetic (+, -, ×, ÷) work with RI. Now let's use RI in a couple of programs.
Memory Cycle
In this program, the user cycles through the contents of R0 through R9. One possible solution is to just line up a batch of RCL N, R/S commands. However, with indirect addressing we can considerably shorten the program by use of registers I and (i).
This program is going to use a loop that increases the counter from 0 to 9. The starting value is 0, the ending value is 9. Let I be the counter variable and I = 0.009.
Hint: STO (i) and RCL (i) uses the integer value of I as the index. (any decimal portion of I is ignored)
Labels used: A (main), 0
Program Listing
Key Listing Keys
001 42 21 11 LBL A
002 48 .
003 0 0
004 0 0
005 9 9
006 44 25 STO I * Stores the Index register
007 42 21 0 LBL 0 * Loop starts here
008 45 24 RCL (i) * Indirect recall
009 31 R/S * Stop and display
010 33 R ↓ * Send value to T-register
011 42 6 25 ISG I
012 22 0 GTO 0
013 43 4 9 SF 9 * Flash the display
014 43 32 RTN
Instructions:
1. Run Program A ( [ f ] [ √ ] (A) )
2. The program starts with R0. Press [R/S] to see the contents of R1. This cycles to R9.
3. When R9 is recalled, the stack is reset and the display flashes. Clear Flag 9 or press the backspace button.
Example:
Try this program with the following registers set as:
R0 = 0
R1 = 25
R2 = 15
R3 = 10
R4 = 50
R5 = 5
R6 = 45
R7 = 30
R8 = 40
R9 = 35
Wheel of Numbers
This program will "spin" the wheel using memory registers R0 through R9. Press R/S to "stop" the wheel.
Labels used: B (main), 1
Program Listing
Key Codes Keys
001 42 21 12 LBL B
002 48 .
003 0 0
004 0 0
005 9 9
006 44 25 STO I
007 42 21 1 LBL 1
008 45 24 RCL (i)
009 42 6 25 ISG I
010 22 1 GTO 1
011 1 1 * If I = 10, subtract 1
012 0 0 * and start over.
013 44 30 25 STO- I
014 22 1 GTO 1
This program produces an endless loop, so when you do run it, be sure to press [R/S] or turn the calculator off.
Instructions:
1. Load the wheel values on registers R0 and R9.
2. Run Program B. ( [ f ] [e^x] (B) ) The calculator will display "running".
3. When you feel lucky, press [R/S]. The value of the wheel's space will be displayed.
Example:
Set the wheel with the following values:
R0 = 0
R1 = 25
R2 = 15
R3 = 10
R4 = 50
R5 = 5
R6 = 45
R7 = 30
R8 = 40
R9 = 35
Run Program B a few times. Did you stop on the 50 space? Good luck!
Hint: With modifications, the above program can simulate a very famous wheel on a very famous game show.
That concludes Part 12. As I previously mentioned, we will continue working with indirect registers in Part 13. Signing off,
Eddie
This tutorial is property of Edward Shore. © 2011