UNDER CONSTRUCTION (may have mistakes).
All of the 65816's special registers have a unique set of capabilities. The two registers most similar to each other are the two index registers X and Y, but uses of them cannot (in general) be substituted for each other without breaking software.
|Register Name||Accumulator||X Index Register||Y Index Register||Stack Pointer||Program Counter||Status|
|can be incremented by one?||INC||INX||INY||PL* when * is an 8-bit register|
|can be decremented by one?||DEC||DEX||DEY||PH* when * is an 8-bit register||BRA|
|can be incremented by two?||ADC||PL* when * is a 16-bit register|
|can be decremented by two?||SBC||PH* when * is a 16-bit register|
|can be loaded with an immediate?||LDA||LDX||LDY|
|can be stored to main memory?||STA||STX||STY|
|can be compared with main memory?||CMP||CPX||CPY|
|can be transferred to A?||¯_(ツ)_/¯||TXA||TYA||TSC|
|can be transferred to X?||TAX||¯_(ツ)_/¯||TYX||TSX|
|can be transferred to Y?||TAY||TXY||¯_(ツ)_/¯|
|can be transferred to S?||TCS||TXS||¯_(ツ)_/¯|