Techniques are presented for sending an application instruction from a hosting digital appliance to a portable medium, where the instruction is structured as one or more units whose size is a first size, or number of bytes. After flushing the contents of a cache, the instruction is written to the cache, where the cache is structured as logical blocks having a size that is a second size that is larger (in terms of number of bytes) than the first size. In writing the instruction (having a command part and, possibly, a data part), the start of the instruction is aligned with one of the logical block boundaries in the cache and the instruction is padded out with dummy data so that it fills an integral number of the cache blocks. When a response from a portable device to an instruction is received at a hosting digital appliance, the cache is similarly flushed prior to receiving the response. The response is then stored to align with a logical block boundary of the cache.