ARM Assembly Language - Introduction [Part 2]

The above article introduces ARM's data transfer instructions. This article will mainly introduce ARM's data processing instructions such as shift, sequence rotation and bit operation.

【Shift instruction】

Shifting includes logical shift and arithmetic shift. The so-called "logical" means that the content stored in the register is just regarded as a string of bits. When shifting, you only need to shift these bits directly to the left (LSL - Logical Shift Left) or right (LSR - Logical Shift Righ). Because the bit width of the register is fixed, some bits will be shifted out of the register.

If you use a circular logical shift instruction such as ROR (Rotate Right), the bits shifted out of the LSB will be moved to the MSB in sequence. Since LSR has a corresponding LSL, does ROR also have a corresponding ROL (Rotate Left)? No, taking a 32-bit system as an example, the effect of circularly shifting left by n bits and circularly shifting right by 32-n bits is actually the same.

Arithmetic, on the other hand, treats the contents of a register as a value, and when shifting, the positive and negative values ​​need to be considered. In ARM, there is only an arithmetic shift right (ASR) instruction, but no arithmetic shift left instruction. In an arithmetic shift right, the vacant bit at the MSB end will be filled with the MSB representing the sign, and each right shift by one bit is equivalent to dividing by 2.

[Data sequence transfer instruction]

"Data reordering" refers to partially swapping bits or bytes in a register. There are many forms of reordering, including the RBIT instruction that reverses all bits, the REV instruction that reverses all bytes, and the REV16 instruction that reverses bytes in two 16-bit half words.

Among them, the REV instruction is probably the most used, because it can be used to convert between Little-Endian (LE) and Big-Endian (BE) in network transmission. In ARMv8, data access can be LE or BE according to the setting of the system control register SCTLR, but instruction fetch is always LE.

On 64-bit systems, a REV32 instruction is added to reverse the bytes in two 32-bit words:

【Bit's Scalpel】

Traditional bit operation instructions such as AND and ORR are frequently used and easy to understand, but if you want to implement some more complex bit operations, you need some more professional instructions, such as the BFI (Bit Field Insert) instruction to insert part of the bits of a register into the specified part of another register, or the BFX (Bit Field Extract) instruction to directly extract these bits, and the BFC (Bit Field Clear) instruction to clear the bits of a specified part.

For example, "BFI W0, W1, #9, #6" means inserting the 6 bits at the LSB end of the W1 register into the W0 register from bit 9 to bit 14.

As I write this, I think of gene editing, and I recommend everyone to read Professor Wang Liming’s "God's Scalpel".

The following will introduce the unconditional jump instructions and conditional jump instructions in ARM.