MM2S_TAILDESC_MSB (MM2S DMA Tail Descriptor Pointer Register – Offset 14h) - 7.1 English

AXI DMA v7.1 LogiCORE IP Product Guide (PG021)

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7.1 English

This register provides the upper 32 bits of Tail Descriptor Pointer for the Memory Map to Stream DMA Scatter Gather Descriptor Management. This is applicable only when the address space is more than 32 bits wide.

Figure 1. MM2S_TAILDESC_MSB Register

Table 1. MM2S_TAILDESC_MSB Register Details
Bits Field Name Default Value Access Type Description
31 to 0 Tail Descriptor Pointer zeros R/W

Indicates the pause pointer in a descriptor chain. The AXI DMA SG Engine pauses descriptor fetching after completing operations on the descriptor whose current descriptor pointer matches the tail descriptor pointer.

When AXI DMA Channel is not halted (DMASR.Halted = 0), a write by the CPU to the TAILDESC_PTR_MSB register causes the AXI DMA SG Engine to start fetching descriptors or restart if it was idle (DMASR.Idle = 1). If it was not idle, writing TAILDESC_PTR has no effect except to reposition the pause point.

If the AXI DMA Channel is halted (DMASR.Halted = 1 and DMACR.RS = 0), a write by the CPU to the TAILDESC_PTR register has no effect except to reposition the pause point.

Note: The software must not move the tail pointer to a location that has not been updated. The software processes and reallocates all completed descriptors (Cmplted = 1), clears the completed bits and then moves the tail pointer. The software must move the pointer to the last descriptor it updated. Descriptors must be 16-word aligned, that is, 0x00, 0x40, 0x80, and so forth. Any other alignment has undefined results.