struc RWBlock
 rwSpecFunc             db 0    ; Special functions (must be zero)
 rwHead                 dw ?    ; Head to read/write
 rwCylinder             dw ?    ; Cylinder to read/write
 rwFirstSector          dw ?    ; First sector to read/write
 rwSectors              dw ?    ; Number of sectors to read/write
 rwBuffer               dd ?    ; Address of buffer for read/write data
ends                            ; = 13d bytes

        ; rwFirstSector: Relative to beginning of track