Buffer Addressing Sample Clauses

Buffer Addressing. The buffer address for an isochronous data packet is determined by using the relative frame number R to pick an Offset or pair of Offsets from the Isochronous TD. These values are used to determine the starting and ending physical address of the buffer for the data packet. Offset[R] determines the starting address. The low order 12 bits of the Offset are the offset within a 4K physical page of the start of the buffer. Bit 12 of offset R then selects the upper 20 bits of the physical address as either BufferPage0 when bit 12 = 0 or the upper 20 bits of BufferEnd when bit 12 = 1. If the data packet is not the last in an Isochronous TD (R not equal to FrameCount), then the ending address of the buffer is found by using Offset[R+1] - 1. This value is then used to create a physical address in the same manner as the Offset[R] was used to create the starting physical address (e.g., use bit 12 as page selector and low order 12 bits as page offset). If, however, the data packet is the last in an Isochronous TD (R = FrameCount), then the value of BufferEnd is the address of the last byte in the buffer. During a data packet transfer, the buffer address may cross a 4K boundary. If this should occur, the HC, as it does with General TDs, uses the upper 20 bits of the computed data packet buffer end address as the physical address of the next page. This allows scatter/gather of the data within a isochronous data packet. If the Host Controller supports checking of the Offsets, if either Offset[R] or Offset[R+1]does not have a ConditionCode of NOT ACCESSED or if the Offset[R+1] is not greater than or equal to Offset[R], then an Unrecoverable Error is indicated.