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cpu-registersmachine-codez80

Z80 register pairs

I am fairly new to the Z80 and machine code, so please don't assume I know anything.

Basically, what I want to know is this: If you load register H with a value (I'll call it y), will HL then be 0xy0? e.g. if H was loaded with 0xAF would HL be 0xAF00? And would the same be true for loading L with y?

Thanks in advance.

Solution

  • The H and L 8-bit registers can be treated independently. Loading a value in H, will not affect the value in L, and vice versa. The two registers H and L can also be treated as a 16-bit register pair. The following source FIRST STEPS IN MACHINE CODE describes this.

    two single register transfers, e.g.

    LD H, B

    LD L, C

    to copy BC into HL.

    and

    You can, if you wish, load a register pair directly with a single instruction, rather than use two instructions. From last time, you will recall that the H and L, B and C, and D and E registers can be paired such that they effectively can hold any number between 0 and 65535 (00 to FFFF hex). C, E, and L form the low byte of the pair, while B, D, and H are the high bytes.