[New] Fast PIO Transfers Other peripherals within the computer, such as video, resolved their throughput problems via local bus architectures providing a potential path for improved performance. IDE local bus solutions, leveraged from the success of video local bus, began appearing in 1992, as a way to enhance data throughput. These solutions mapped the IDE data port to the local bus, bypassing the ISA bus and enabling the maximization of throughput from the media to the drive buffer, on to the host. These solutions were still not competitive with Fast SCSI (10MB/sec) due to the "blind" transfer nature of the PIO transfers . "Blind PIO" transfers indicate host control of data throughput with the host requesting data (master) and the drive responding (slave). With blind PIO transfers, the host is unaware or "blind" when buffered drive bandwidth is 100% available for host transfers. Because there are cases when only a percentage of bandwidth is available, blind PIO host requests for data from the drive are based on the worst case bandwidth availability. This means that even when the ISA bottleneck is bypassed by connection directly to the local bus, inability to utilize 100% drive bandwidth prevents full optimization of host throughput. Enhanced IDE incorporates an operation called "Flow Control Using IORDY" (I/O Channel Ready) which allows the drive to "throttle" the host when necessary and enable burst transfers to take advantage of 100% of the bandwidth. Flow Control thereby gives control of the data transfer to the drive and eliminates the inefficiencies of blind PIO by setting the host to maximum drive bandwidth support. This means that when 100% drive bandwidth is available, the drive will take control and transfer data to the host. This operation, based on approved Mode 3 PIO timings of 180ns cycle times from the Small Form Factor Committee, supports transfer rates up to 11MB/sec competitive with FAST SCSI solutions. Flow Control is enabled on the drive by the host issuing a Set Features command, so that both the host and drive side support this operation. Western Digital's 540MB drives (shipping beginning September, 1993) support flow control using IORDY and will be implemented into machines that take advantage of this feature via low cost ASICS whose functionality will later be incorporated into core logic chipset solutions. DMA Transfers Although PIO is the standard transfer method supported by the industry and presents no incompatibility issues (see footnote), another transfer option exists that provides incremental transfer benefits beyond PIO. Direct Memory Access (DMA) is based on data transfer directly to memory rather than via the CPU. DMA transfers are "throttled" and therefore have historically offered the benefit of maximizing data throughput. The throttling mechanism associated with DMA has historically enabled improved data transfers relative to standard PIO. Type B DMA was defined with the arrival of Extended Industry Standard Architecture (EISA), and is specified at 4.0MB/sec transfer rates offering an advantage to the standard 2-3MB/sec PIO data rates. Although this is an improvement to the standard ISA bus timings, Type B DMA remains uncompetitive with FAST SCSI timings of 10MB/sec. With the advent of local bus solutions, a new DMA transfer has emerged in conjuction with PCI. Type F DMA is defined to support 8.33MB/sec and 6.67MB/sec data rates, a large improvement over Type B DMA. In conjunction with chipsets capable of supporting 6.67MB and 8.33MB/sec data rates, the Small Form Factor Committee has approved a new multiword DMA Mode 1 timing specification of a 150ns cycle time. This enables DMA transfers up to 13MB/sec for future data rate improvement by allowing multiple words to be transferred for any given request command. PCI chip sets will be shipping with both EISA (Type B) and ISA (Type F) configurations in the calendar CYQ4'1993 time frame. --------------------------------------------------------------------------- [ ] [ Home | New | Company | Products | Where2Buy | Service | Coolstuff | Promos | Partners | Employment | Index | Search ] Many Western Digital documents are available in Acrobat format. ⌐Copyright 1994, 1995, 1996 Western Digital Corporation. All rights reserved. Problems or comments? Please contact webmaster@www.wdc.com