Memory device and host device

A method of controlling a memory card, the memory card including a nonvolatile semiconductor memory having a plurality of physical storage areas that include a user area externally accessible, the user area being divided into plurality of allocation units,

• the method comprising:
• receiving a control command having a first argument to designate a sequential write area and a read command or a write command;
• assigning an allocation unit of the plurality of allocation units represented by an address of the read command or the write command as the sequential write area;
• and
• changing memory access control when an address of a memory access command to access the user area indicates no access in the sequential write area whose size is equivalent to the allocation unit, wherein
• the allocation unit of the plurality of allocation units is divided into plurality of recording units,
• the memory access control includes controlling a busy time according to a data length of the write command and one or plurality of recording unit, and
• when the address of the write command for the sequential write area is an address not to cause overwrite, the data length of the write command is equivalent to one or plurality of recording unit and a data location is on a recording unit boundary, the method further including:
• controlling a busy time to make average write performance higher than a performance index, and if the condition is not met, indicating a busy time that does not meet the performance index.