General Registers (AX, BX, CX and DX)

The registers AX, BX, CX, and DX behave as general purpose registers in Intel architecture and do some specific functions in addition to it. X in their names stand for extended meaning 16bit registers. For example AX means we are referring to the extended 16bit “A” register. Its upper and lower byte are separately accessible as AH (A high byte) and AL (A low byte). All general purpose registers can be accessed as one 16bit register or as two 8bit registers. The two registers AH and AL are part of the big whole AX. Any change in AH or AL is reflected in AX as well. AX is a composite or extended register formed by gluing together the two parts AH and AL.

The A of AX stands for Accumulator. Even though all general purpose registers can act as accumulator in most instructions there are some specific variations which can only work on AX which is why it is named the accumulator. The B of BX stands for Base because of its role in memory addressing as discussed in the next chapter. The C of CX stands for Counter as there are certain instructions that work with an automatic count in the CX register. The D of DX stands for Destination as it acts as the destination in I/O operations. The A, B, C, and D are in letter sequence as well as depict some special functionality of the register.

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