[SOLVED] How to increment 64-bit memory effective address in Real Mode

How to increment 64-bit memory effective address in Real Mode

I am reading sectors from my extended drive in Real Mode using the interrupt 0x13 with the function of extended drives 0x42.

I define DAP to be 16 bytes in the following structure:

DAP:
    db  0x10        ; size of DAP
    db  0           ; Reserved zero
    dw  0x0001      ; Number of sectors to read
    dd  0x00000200  ; Memory Location to load the sector (s)
    dq  0           ; Start of the sectors to be read

The DAP segment is of 8-bytes length as you could notice. During seeking for my sector (looping over sectors), I increment the segment and compare it to the real size of my drive. The wrong code that I am using to increment is limited to 16-bit mode:

mov     ax, [DAP+0x08]
inc     ax
mov     [DAP+0x08], ax

I don't want to use several general purpose registers in a complicated addressing mode to achieve my purpose, I guess you have some simple and efficient way.

Solution

To increment a 64-bit QWORD, you can use the add and adc instructions:

ADD WORD [DAP+ 8], 1
ADC WORD [DAP+10], 0
ADC WORD [DAP+12], 0
ADC WORD [DAP+14], 0

Or, if you aren't targeting an 8088, 8086, or 80286, you can also use a 32-bit add/adc:

ADD DWORD [DAP+ 8], 1
ADC DWORD [DAP+12], 0

Note that you can't use INC WORD [DAP+ 8] instead of ADD WORD [DAP+ 8], 1 because the former doesn't set the carry flag.