AMD SC2200 manual DMA Transfer Modes, DMA Controller Registers, DMA Transfer Types, DMA Priority

Core Logic Module 32580B

DMA Transfer Modes

Each DMA channel can be programmed for single, block, demand or cascade transfer modes. In the most commonly used mode, single transfer mode, one DMA cycle occurs per DRQ and the PCI bus is released after every cycle. This allows the Core Logic module to timeshare the PCI bus with the GX1 module. This is imperative, especially in cases involving large data transfers, because the GX1 module gets locked out for too long.

In block transfer mode, the DMA controller executes all of its transfers consecutively without releasing the PCI bus.

In demand transfer mode, DMA transfer cycles continue to occur as long as DRQ is high or terminal count is not reached. In this mode, the DMA controller continues to exe- cute transfer cycles until the I/O device drops DRQ to indi- cate its inability to continue providing data. For this case, the PCI bus is held by the Core Logic module until a break in the transfers occurs.

In cascade mode, the channel is connected to another DMA controller or to an ISA bus master, rather than to an I/O device. In the Core Logic module, one of the 8237 con- trollers is designated as the master and the other as the slave. The HOLD output of the slave is tied to the DRQ0 input of the master (Channel 4), and the master’s DACK0# output is tied to the slave’s HLDA input.

In each of these modes, the DMA controller can be pro- grammed for read, write, or verify transfers.

Both DMA controllers are reset at power-on reset (POR) to fixed priority. Since master Channel 0 is actually connected to the slave DMA controller, the slave’s four DMA channels have the highest priority, with Channel 0 as highest and Channel 3 as the lowest. Immediately following slave Channel 3, master Channel 1 (Channel 5) is the next high- est, followed by Channels 6 and 7.

DMA Controller Registers

The DMA controller can be programmed with standard I/O cycles to the standard register space for DMA. The I/O addresses for the DMA controller registers are listed Table 6-43 on page 305.

When writing to a channel's address or WORD Count reg- ister, the data is written into both the base register and the current register simultaneously. When reading a channel address or WORD Count register, only the current address or WORD Count can be read. The base address and base WORD Count are not accessible for reading.

DMA Transfer Types

Each of the seven DMA channels may be programmed to perform one of three types of transfers: read, write, or ver- ify. The transfer type selected defines the method used to transfer a byte or WORD during one DMA bus cycle.

For read transfer types, the Core Logic module reads data from memory and write it to the I/O device associated with the DMA channel.

For write transfer types, the Core Logic module reads data from the I/O device associated with the DMA channel and write to the memory.

The verify transfer type causes the Core Logic module to execute DMA transfer bus cycles, including generation of memory addresses, but neither the READ nor WRITE com- mand lines are activated. This transfer type was used by DMA Channel 0 to implement DRAM refresh in the original IBM PC and XT.

DMA Priority

The DMA controller may be programmed for two types of priority schemes: fixed and rotate (I/O Ports 008h[4] and 0D0h[4] - see Table 6-43 on page 305).

In fixed priority, the channels are fixed in priority order based on the descending values of their numbers. Thus, Channel 0 has the highest priority. In rotate priority, the last channel to get service becomes the lowest-priority channel with the priority of the others rotating accordingly. This pre- vents a channel from dominating the system.

The address and WORD Count registers for each channel are 16-bit registers. The value on the data bus is written into the upper byte or lower byte, depending on the state of the internal addressing byte pointer. This pointer can be cleared by the Clear Byte Pointer command. After this com- mand, the first read/write to an address or WORD-count register reads or writes to the low byte of the 16-bit register and the byte pointer points to the high byte. The next read/write to an address or WORD-count register reads or writes to the high byte of the 16-bit register and the byte pointer points back to the low byte.

When programming the 16-bit channels (Channels 5, 6, and 7), the address which is written to the base address register must be the real address divided by two. Also, the base WORD Count for the 16-bit channels is the number of 16-bit WORDs to be transferred, not the number of bytes as is the case for the 8-bit channels.

The DMA controller allows the user to program the active level (low or high) of the DRQ and DACK# signals. Since the two controllers are cascaded together internally on the chip, these signals should always be programmed with the DRQ signal active high and the DACK# signal active low.

DMA Shadow Registers

The Core Logic module contains a shadow register located at F0 Index B8h (Table 6-29 on page 198) for reading the configuration of the DMA controllers. This read only regis- ter can sequence to read through all of the DMA registers.

AMD Geode™ SC2200 Processor Data Book 161