HP 50g Graphing manual Preserve Save and Restore Previous Status, PAD program listing, `Opad K

PAD program listing

		Program:	Comments:
		«
		→STR	Makes sure the object is in string form. (Strings
			are unaffected by this command.)
		WHILE	Repeats if the string contains fewer than 22
		DUP SIZE 22 <	characters.
		REPEAT	Loopclause adds a leading space.
		" " SWAP +
		END	End loop.
		»
		`OPAD K	Stores the program in PAD.
Checksum: # 6577d
Bytes:	57.5

PAD is demonstrated in the program BDISP.

PRESERVE (Save and Restore Previous Status)

PRESERVE stores the current calculator (flag) status, executes a program from the stack, and restores the previous status.

Level 1	→	Level 1
«program»	→	result of program
'program'	→	result of program

Techniques used in PRESERVE

Preserving calculator flag status. PRESERVE uses RCLF (recall flags) to record the current status of the calculator in a binary integer, and STOF (store flags) to restore the status from that binary integer.

Localvariable structure. PRESERVE creates a local variable structure to briefly remove the binary integer from the stack. Its defining procedure simply evaluates the program argument, then puts the binary integer back on the stack and executes STOF.

Error trapping. PRESERVE uses IFERR to trap faulty program execution on the stack and to restore flags. DOERR shows the error if one occurs.

26 RPL Programming Examples