Chipmunk is a minimized 6502 so it's missing some features, and there are some new ones that weren't available on the 6502.

Missing opcodes
  * BIT
  * ORA         (available as a macro)
  * AND         (available as a macro for constants only)
  * TAY/TAY     (available as a macro)
  * CLI/SEI/RTI (no interrupts)
  * TXS/TSX
  * CLV/BVC/BVS (no overflow flag)
  * BRK
  * SED/CLD     (no decimal mode)

New opcodes (compared to NMOS 6502)
  * SWAPAX  - swap the values of A and X (SWAPAY and SWAPXY available as macros)
  * BIC     - (macro) accumulator = accumulator & ~value
  * NEG     - (macro) accumulator = -accumulator
  * NOT     - accumulator = ~accumulator
  * ADD     - add without carry
  * SUB     - subtract without carry
  * INA     - increment accumulator
  * DEA     - decrement accumulator
  * NOR     - accumulator = ~(accumulator | value)
  * BRA     - branch always
  * BSR     - call subroutine, but relative
  * JEQ/JNE - jump if equal/not equal
  * JCS/JCC - jump if carry set/unset
  * JMI/JPL - jump if negative/positive
  * ADX/SBX - add and subtract constants from the X register
  * ADY/SBY - add and subtract constants from the Y register
  * PHX/PLX - push/pull X register
  * PHY/PLY - push/pull Y register
  * HALT    - stops the coprocessor, signals that it's done

Missing address modes
  * Zeropage, x
  * (Zeropage, x)
  * Absolute, Y   (except for LDA/STA)
  * (Zeropage), y (except for LDA/STA)

* 6-bit stack pointer, though that can be changed.
* If an address or constant is zero, it takes no space to store.
* No page crossing penalties.
* Branches can take a 16-bit parameter instead and act as a jump.
* ASL/LSR only work on the accumulator.
* ROL/ROR only work on memory.
* TAX/TXA don't affect flags.