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2.2.5. Scatter-gather

When scatter-gather mode is enabled, the TMC uses tables in memory to find the addresses of pages of memory to use for trace storage.All pages are 4KB in size, and each is referenced by a single entry in the tables. The tables form a linked list, to ensure that the tables themselves not required to be larger than 4KB.

Figure 2.6 shows the format of the scatter-gather tables.

Figure 2.6. Scatter-gather description

Figure 2.6. Scatter-gather description

Table 2.3 shows the format of the 32-bit entries used in this table.

Table 2.3. Page table 0 entries during scatter-gather operation
BitsDescription
[31:4]Bits [39:12] of an address. Bits [11:0] are always zero. The meaning of this field is dependent on the entry type.
[3:2]Reserved, SBZ
[1:0]

Entry type:

00

Reserved

01

Last. The address points to a 4KB page to use for buffer storage, and this is the last entry in the tables.

10

Normal. The address points to a 4KB page to use for buffer storage, and the next entry in the table is valid. The entry must not be the last entry in a 4KB page.

11

Link. The address points to the base of a further 4KB page of table entries.


The TMC fetches each page table entry while the previous page is being written to, so that page table reads do not impact performance. When the last entry is reached, the TMC fetches the first entry from the first page table again.

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