Apparatus and method for reducing read-modify-write cycles by combining unaligned write commands

Write commands for a storage device specify write data with either a first data step size or a second data step size. In the former case, the storage device performs a read-modify-write (RMW) cycle which includes reading data with the second data step size. In the latter case, the storage device executes the command in a single write cycle. A command sorting unit sorts received commands into two groups, first commands and second commands, when storing them in a memory. First commands are write commands whose data boundaries do not match with the second data step size. Second commands include write commands whose data boundaries match with the second data step size. A command issuing unit converts first commands into a second command upon predetermined conditions. The command issuing unit issues the second commands to the storage device, in preference to the first commands.