push eax

Output:

50

Which is a single opcode.

The opcode can be further decomposed into the following bits:

0 1 0 1 0 0 0 0
^^^^^^^^^ ^^^^^
1         2

1. This is a push instruction.
2. From where we will push. 000 is eax.

This is documented as: opcode == 50+rd in the Intel manual. The +rd part says that the 3 last bits indicate where to push from.