Based on Bill Verners' Inside the Java Virtual Machine, McGraw Hill Companies, 1997

pop - Pop top stack word
  opcode: 87 (0x57)
  usage: pop
stack before: ..., word
stack after: ...
 
description: This instruction pops one word from the stack.

pop2 - Pop top two stack words
  opcode:  88 (0x58)
  usage: pop2
  stack before: ..., word2, word1
stack after: ...
 
description: This instruction pops two words from the stack.

putfield - Set field in object
  opcode:  181 (0xb5)
  usage: putfield field-spec descriptor
  stack before: ..., objectref, value
stack after: ...

or

stack before: ..., objectref, value.word1, value.word2
stack after: ...

 
description: This instruction puts value into a nonstatic field of the object on the top of the stack. field-spec is composed of two parts, a classname and field name (f.e. foo/bar/ClassName/fieldName). descriptor is the Java type descriptor of the field.

putstatic - Set static field in class
  opcode:  179 (0xb3)
  usage: putstatic field-spec descriptor
  stack before: ..., value
stack after: ...

or

stack before: ..., value.word1, value.word2
stack after: ...

 
description: This instruction puts value into a static field of the object on the top of the stack. field-spec is composed of two parts, a classname and field name (f.e. foo/bar/ClassName/fieldName). descriptor is the Java type descriptor of the field.