多個PowerPC指令具有相同的操作碼

Question

我試圖理解PowerPC指令並最終反彙編它們。我發現http://pic.dhe.ibm.com/infocenter/aix/v7r1/index.jsp?topic=/com.ibm.aix.aixassem/doc/alangref/ppc_instr.htm，而且我很困惑 - addic和si如何具有相同的操作碼？

Answer 1

顯然它是彙編器部分的某種語法糖。他們都執行加法，而是si用負數做的：

The si instruction subtracts the 16-bit signed integer specified by the 
SINT parameter from the contents of general-purpose register (GPR) RA and 
stores the result in the target GPR RT. This instruction has the same 
effect as the ai instruction used with a negative SINT value. The 
assembler negates SINT and places this value (SI) in the machine 
instruction: 

    ai RT,RA,-SINT 

來源：si (Subtract Immediate) Instruction

Answer 2

它們是相同的指令。不同之處在於，在si的情況下，彙編程序更改提供的立即數的符號。