It turns out that there are an extra four data memories or an extra 32 program steps in the basic TI-58C. Note that this only applies to the TI-58C and NOT the TI-58.
The data memory register 59 corresponding to program steps 480 to 487 is used by the TI-58C's constant memory feature to set the partition between data and program memories, to set the FIX N display, and to check for memory loss at turn on. By changing the contents of this memory, the TI-58C can be repartitioned so that these 32 steps/4 memories are directly accessible.
The first 13 digits of this memory 59 store an unscaled LN(10) value. If this number is not present at turn on, the calculator erases all data and program memory to protect the user from unsuspected memory loss. Instead, the user suffers from known memory loss!
The next two digits of the memory (the first digit of step 480 and the last digit of step 481) set the partition at turn on. By writing the proper key codes into these steps, the TI-58C can be partitioned like a TI-59 or entirely new partitions can be created.
First, however, steps 480-481 must be accessed. To do this, write in program memory: LBL A FIX 0 R/S. Then enter the number 609.000000609 into the display and press A. (Enter this value by typing 0.000000609 then adding 609 to it. Another example of using the TI-58/59's greater than 10 digit precision). Then press STF IND 7 INV LRN. You should be at step 480. Key in the instructions GRAD and RCL into steps 480-481 in program mode. Press LRN to leave program mode. Then cycle the on-off switch. Press OP 16 and you should see a partition of 159.99, meaning that you have 100 memories and have 160 program steps.
Once this is done, you may repartition the TI-58C for any of the normal TI-59 partitions. For example, a partition of 6 OP 17 provides the display 479.59 and gives the TI-58C 480 program steps and 4 hardwired memories available in addressed as STO 56, STO 57, STO 58 and STO 59.
You must be careful with memory 59 (since that will correspond to step 480 and 481 as part of that memory). Changing the display setting with a FIX command will alter parts of this data memory and LN(10) must be stored there prior to turning off the machine to avoid a memory lost. For that reason, I suggest you leave memory 59 alone!
This would allow you to write a very long program and be able to use 3 memories. The program would otherwise not fit in a TI-58C.
Note: When you turn the machine on and off, any pseudo-TI-59 partitioning scheme will revert back to a "primary" partition. You would need to redo an OP 17 to re-establish the previous partitioning when you turn the machine back on.
Other useful TI-59 partitions would include:
6 OP 17 - 480 program steps and 3 memories (56, 57, and 58)
7 OP 17 - 400 program steps and 13 memories (0-9, 56, 57, 58)
8 OP 17 - 320 program steps and 23 memories (0-19, 56, 57, 58)
And so forth.
You don't really gain anything with partitions less than 6 OP 17.
Enjoy this particular quirk.