✎ javacard:jcvm:7.2_reserved_opcodes [wiki]

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===== 7.2 Reserved Opcodes =====

In addition to the opcodes of the instructions specified later this chapter, which are used in Java Card CAP files (see [[6._The_CAP_File_Format| Chapter 6]], "The CAP File Format"), two opcodes are reserved for internal use by a Java Card virtual machine implementation. If Sun Microsystems, Inc. extends the instruction set of the Java Card virtual machine in the future, these reserved opcodes are guaranteed not to be used. 

The two reserved opcodes, numbers 254 (0xfe) and 255 (0xff), have the mnemonics impdep1 and impdep2, respectively. These instructions are intended to provide "back doors" or traps to implementation-specific functionality implemented in software and hardware, respectively. 

Although these opcodes have been reserved, they may only be used inside a Java Card virtual machine implementation. They cannot appear in valid CAP files.

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