The TI-88 calculator contains a large number of functions accessible through the OP key. The manual lists them all, but there appears to be a slight discrepancy between the manual and the OP codes on the prototype that's in my possession. Here's a list of all TI-88 OP codes that work on the prototype:

OP 00   List OP code definitions
OP 01   Set calculator to default operating modes
OP 02   Display calculator settings
OP 03   Recall last error message
OP 04   YES/NO/UNK/ENT/CONT cue instruction for programs
OP 05   YES/NO cue instruction for programs
OP 06   ENT/CONT cue instruction for programs
OP 07   CONT cue instruction for programs
OP 08   List ALPH key second functions
OP 09   Recall alpha display register
OP 10   Right circular shift of alpha register
OP 11   Left circular shift of alpha register
OP 12   Show all 13 digits
OP 13   Round display register
OP 14   Unformatted display mode of 16 BCD digits
OP 15   Cancel unformatted mode
OP 16   Hexadecimal mode
OP 17   Decimal mode
OP 18   Display system flag definitions
OP 19   Display flags that are set
OP 20   Save user flags into an internal hierarchy register
OP 21   Exchange saved and current user flags
OP 22   Set pause timing to default
OP 23   Set pause timing
OP 24   Turn on implied multiplication mode
OP 25   Cancel implied multiplicatio mode
OP 26   Absolute value function
OP 27   Signum function
OP 28   Convert D.MMSS to decimal degrees
OP 29   Convert decimal degrees to D.MMSS
OP 30   Display current angle mode
OP 31   Convert degrees to radians
OP 32   Convert radians to degrees
OP 33   Convert radians to grads
OP 34   Convert grads to radians
OP 35   Convert grads to degrees
OP 36   Convert degrees to grads
OP 37   Clear statistical registers
OP 38   Compute Y-intercept and slope
OP 39   Compute correlation coefficient
OP 40   Compute Y estimate
OP 41   Compute X estimate
OP 42   Compute statistical means
OP 43   Display number of data entries
OP 44   Compute standard deviation (N)
OP 45   Compute standard deviation (N-1)
OP 46   Set program counter to display value
OP 47   Copy program instruction code to display
OP 48   Copy display to program instruction
OP 49   Set default partitioning
OP 50   Set partitioning
OP 51   Soft partitioning
OP 52   Hard partitioning
OP 53   List program labels
OP 54   Calculator system test 1
OP 55   Calculator system test 2
OP 56   Read tape file into main memory
OP 57   Write main memory contents to tape
OP 58   Read tape file into program memory
OP 59   Write program memory contents to tape
OP 60   Read tape file into data memory
OP 61   Write data memory contents to tape
OP 62   Read numbered module file from tape
OP 63   Write numbered module file to tape
OP 64   Convert decimal to hexadecimal
OP 65   Convert hexadecimal to decimal
OP 66   Test module number
OP 67   Test module status
OP 68   Number the module in the primary port
OP 69   Erase the module in the primary port
OP 70   Read module program into program memory
OP 71   Write program memory contents to module
OP 72   Protect module
OP 73   Copy module
OP 74   24-hour mode
OP 75   12-hour mode
OP 76   Add time values in HH.MMSS form
OP 77   Subtract time values in HH.MMSS form
OP 78   Set alarm time
OP 79   Turn alarm on
OP 80   Turn alarm off
OP 81   Sound a tone
OP 82   Turn on tone on error
OP 83   Turn off tone on error
OP 84   Turn on tone on cue prompt
OP 85   Turn off tone on cue prompt
OP 86   Output data*
OP 87   Input data

*The manual lists OP 86 as an instruction that turns on keyboard tones. However, on the actual prototype this instruction is omitted, and the remaining two OP codes are shifted down by one.