ARM匯編中的立即數(shù)

------------------------------------我是無辜的分割線--------------------------------

在 mov r2,#0xffb 這句中，不是MOV的用法出錯，而是立即數(shù)用法出錯。
立即數(shù)的用法定義在Arm Architechture Reference Manual(簡稱ARMARM)的A5-4頁開始

很重要的一段：

An immdediate operand value is formed by rotating an 8-bit constant (in a 32-bit word) by an even number of bits (0,2,4,8,26,28,30). Therefore, each instruction contains an 8-bit constant and a 4-bit rotate to be applied to that constant.

Some valid constants are:
0xFF, 0x104, 0xFF0, 0xFF00, 0xFF000, 0xFF000000, 0xF000000F

Some invalid constants are:
0x101, 0x102, 0xFF1, 0xFF04, 0xFF003, 0xFFFFFFFF, 0xF000001F

而在下面的A5-6頁中提到
Specifies the immediate constant wanted. It is encoded in the instruction as an 8-bit immediate (immed_8) and a 4-bit immediate (rotate_imm), so that is equal to the result of rotating immed_8 right by (2*rotate_imm) bits.

shifter_operand = immed_8 Rotate_Right (rotate_imm * 2)

以及
Some values of have more than one possible encoding. When more than one encoding is available, an assembler needs to choose the correct one to use. For more precise control of the encoding, the instruction fields can be specified directly byusing the syntax: #,

所以，綜上所述，首先解釋清楚了 mov r2,#0xFF,28 一句。28并不是第三個操作數(shù)，而是和0xFF并在一起作為立即數(shù)使用，將0xFF循環(huán)右移28位。而這里必須強(qiáng)調(diào)右移XX位必須是個偶數(shù)，因為它將等于 rotate_imm*2，那么在該指令的機(jī)器碼中rotate_imm = 14, 也就是在32-BIT的機(jī)器碼中第11到第8 bit = 1110B

然后再來看 mov r2,#0xFFB 的出錯情況
0xFFB = 111111111011B，很顯然按照 shifter_operand = immed_8 Rotate_Right (rotate_imm * 2) 的公式， shifter_operand = 0xFFB時無法得到有效的 immed_8 和 rotate_imm, 所以編譯出現(xiàn)錯誤 error A0092: no immediate rotate operand can be created: 4091 也可以理解了，它應(yīng)該是說無法生成rotate_imm，實際上immed_8也是無法生成的。

關(guān)于立即數(shù)如何分解成immed_8和rotate_imm，可以參考上面給出的valid constants和invalid constants，簡而言之，如果該立即數(shù)可以分解成一個8-bit的二進(jìn)制數(shù)循環(huán)右移偶數(shù)位，那么這個立即數(shù)是有效的，反之無效。

在上面的例子中，想要得到 r2 = 0xFFB 那么在匯編里就必須走兩步了，一步是無論如何無法到達(dá)的。