Editing Direct memory access (section)

=== ISA ===
In the original [[IBM PC]] (and the follow-up [[PC/XT]]), there was only one [[Intel 8237]] DMA controller capable of providing four DMA channels (numbered 0–3). These DMA channels performed 8-bit transfers (as the 8237 was an 8-bit device, ideally matched to the PC's [[i8088]] CPU/bus architecture), could only address the first ([[i8086]]/8088-standard) megabyte of RAM, and were limited to addressing single 64&nbsp;[[kilobyte|kB]] segments within that space (although the source and destination channels could address different segments). Additionally, the controller could only be used for transfers to, from or between expansion bus I/O devices, as the 8237 could only perform memory-to-memory transfers using channels 0 & 1, of which channel 0 in the PC (& XT) was dedicated to [[dynamic memory]] [[memory refresh|refresh]]. This prevented it from being used as a general-purpose "[[Blitter]]", and consequently block memory moves in the PC, limited by the general PIO speed of the CPU, were very slow.

With the [[IBM PC/AT]], the enhanced [[AT bus]] (more familiarly retronymed as the [[Industry Standard Architecture]] (ISA)) added a second 8237 DMA controller to provide three additional, and as highlighted by resource clashes with the XT's additional expandability over the original PC, much-needed channels (5–7; channel 4 is used as a cascade to the first 8237). ISA DMA's extended 24-bit address bus width allows it to access up to 16&nbsp;MB lower memory.<ref>{{Cite web |title=ISA DMA - OSDev Wiki |url=https://wiki.osdev.org/ISA_DMA |access-date=2025-04-20 |website=wiki.osdev.org}}</ref> The page register was also rewired to address the full 16 MB memory address space of the 80286 CPU. This second controller was also integrated in a way capable of performing 16-bit transfers when an I/O device is used as the data source and/or destination (as it actually only processes data itself for memory-to-memory transfers, otherwise simply ''controlling'' the data flow between other parts of the 16-bit system, making its own data bus width relatively immaterial), doubling data throughput when the upper three channels are used. For compatibility, the lower four DMA channels were still limited to 8-bit transfers only, and whilst memory-to-memory transfers were now technically possible due to the freeing up of channel 0 from having to handle DRAM refresh, from a practical standpoint they were of limited value because of the controller's consequent low throughput compared to what the CPU could now achieve (i.e., a 16-bit, more optimised [[80286]] running at a minimum of 6&nbsp;MHz, vs an 8-bit controller locked at 4.77&nbsp;MHz). In both cases, the 64&nbsp;kB [[x86 memory segmentation|segment boundary]] issue remained, with individual transfers unable to cross segments (instead "wrapping around" to the start of the same segment) even in 16-bit mode, although this was in practice more a problem of programming complexity than performance as the continued need for DRAM refresh (however handled) to monopolise the bus approximately every 15&nbsp;[[μs]] prevented use of large (and fast, but uninterruptible) block transfers.

Due to their lagging performance (1.6&nbsp;[[megabyte|MB]]/s maximum 8-bit transfer capability at 5&nbsp;MHz,<ref name="i8237sheet">{{cite web |title=Intel 8237 & 8237-2 Datasheet |url=http://www.jbox.dk/rc702/hardware/intel-8237.pdf |website=JKbox RC702 subsite |access-date=20 April 2019}}</ref> but no more than 0.9&nbsp;MB/s in the PC/XT and 1.6&nbsp;MB/s for 16-bit transfers in the AT due to ISA bus overheads and other interference such as memory refresh interruptions<ref name="DMAfundamentals">{{cite web |title=DMA Fundamentals on various PC platforms, National Instruments, pages 6 & 7 |url=https://cires1.colorado.edu/jimenez-group/QAMSResources/Docs/DMAFundamentals.pdf |access-date=26 April 2025 |website=University of Colorado Boulder}}</ref>) and unavailability of any speed grades that would allow installation of direct replacements operating at speeds higher than the original PC's standard 4.77&nbsp;MHz clock, these devices have been effectively obsolete since the late 1980s. Particularly, the advent of the [[80386]] processor in 1985 and its capacity for 32-bit transfers (although great improvements in the efficiency of address calculation and block memory moves in Intel CPUs after the [[80186]] meant that PIO transfers even by the 16-bit-bus [[80286|286]] and [[80386SX|386SX]] could still easily outstrip the 8237), as well as the development of further evolutions to ([[Extended Industry Standard Architecture|EISA]]) or replacements for ([[Micro Channel architecture|MCA]], [[VESA local bus|VLB]] and [[Peripheral Component Interconnect|PCI]]) the "ISA" bus with their own much higher-performance DMA subsystems (up to a maximum of 33&nbsp;MB/s for EISA, 40&nbsp;MB/s MCA, typically 133&nbsp;MB/s VLB/PCI) made the original DMA controllers seem more of a performance millstone than a booster. They were supported to the extent they are required to support built-in legacy PC hardware on later machines. The pieces of legacy hardware that continued to use ISA DMA after 32-bit expansion buses became common were [[Sound Blaster]] cards that needed to maintain full hardware compatibility with the [[Sound Blaster standard]]; and [[Super I/O]] devices on motherboards that often integrated a built-in [[floppy disk]] controller, an [[IrDA]] infrared controller when FIR (fast infrared) mode is selected, and an [[IEEE 1284]] parallel port controller when ECP mode is selected. In cases where an original 8237s or direct compatibles were still used, transfer to or from these devices may still be limited to the first 16&nbsp;MB of main [[RAM]] regardless of the system's actual address space or amount of installed memory.

Each DMA channel has a 16-bit address register and a 16-bit count register associated with it. To initiate a data transfer the device driver sets up the DMA channel's address and count registers together with the direction of the data transfer, read or write. It then instructs the DMA hardware to begin the transfer. When the transfer is complete, the device [[interrupt]]s the CPU.

Scatter-gather or [[vectored I/O]] DMA allows the transfer of data to and from multiple memory areas in a single DMA transaction. It is equivalent to the chaining together of multiple simple DMA requests. The motivation is to off-load multiple [[input/output]] interrupt and data copy tasks from the CPU.

DRQ stands for ''Data request''; DACK for ''Data acknowledge''. These symbols, seen on hardware [[schematic]]s of computer systems with DMA functionality, represent electronic signaling lines between the CPU and DMA controller. Each DMA channel has one Request and one Acknowledge line. A device that uses DMA must be configured to use both lines of the assigned DMA channel.

16-bit ISA permitted bus mastering.<ref>{{Citation |title=PC Architecture for Technicians: Level 1 |contribution=Chapter 12: ISA Bus |contribution-url=http://faculty.chemeketa.edu/csekafet/elt256/pcarch-full_isa-bus.pdf |author=Intel Corp. |date=2003-04-25 |access-date=2015-01-27}}</ref>

Standard ISA DMA assignments:{{cn|date=November 2024}}

{{ordered list|start=0
| [[DRAM]] refresh (obsolete)
| User hardware usually ISA sound card
| [[Floppy disk]] controller
| [[WDMA (computer)|WDMA]] for [[hard disk]] controller (replaced by [[UDMA]] modes), parallel port (ECP capable port), or certain SoundBlaster Clones like the OPTi 928
| [[8237]] DMA controller
| Hard disk controller ([[PS/2]] only), or user hardware usually ISA sound card
| User hardware
| User hardware
}}