Why jvm expands byte & short to int before pushing on stack?

Question

java code : ,

byte a_b = 12;
short c_d = 14

replaces in bytecodes with

bipush  12 // expands byte1 (a byte type) to an int and pushes it onto the stack
sipush   14 // expands byte1, byte2 (a short type) to an int and pushes it onto the stack

Why jvm does that expansion, and not use byte & short ?

Also when i open bytecode of my file

EDIT : short var = 14 is replaced by bipush 14 rather than sipush 14

Is my understanding is not clear or is there a bug ?

I am using following version

java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) Client VM (build 20.1-b02, mixed mode, sharing)

Answer 1

Because (conceptually) the smallest unit of data on the JVM stack is 32 bits. So there is no way to increase the size of the stack with just 8 bits.

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6.2

At any point in time, an operand stack has an associated depth, where a value of type long or double contributes two units to the depth and a value of any other type contributes one unit.