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Browse Prior Art Database

DMA ROOT POINTER

IP.com Disclosure Number: IPCOM000005624D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2001-Oct-22
Document File: 1 page(s) / 65K

Publishing Venue

Motorola

Related People

Brad Cohen: AUTHOR [+2]

Abstract

Logical bus DMA activity is supported by the MC68851 PMMU. Designers use logical DMA to gain access to the protection mechanisms provided by a memory management unit. When a logical bus access occurs, the PMMU references its Address Translation Cache (ATC) and drives out the physical address translation associated with the logical access. In parallel, the PMMU also performs checks on protection information stored along with the physical address, and asserts control signals for either a normal continuation or exceptional termina- tion of the bus cycle. If, however, there is no entry in the PMMU ATC which matches the logical access, the PMMU must perform a tablewalk to fetch the translation and protection information from its tree-like transla- tion tables.

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MOTOROLA Technical Developments Volume 6 October 1986

DMA ROOT POINTER

by Brad Cohen and Bill Moyer

   Logical bus DMA activity is supported by the MC68851 PMMU. Designers use logical DMA to gain access to the protection mechanisms provided by a memory management unit. When a logical bus access occurs, the PMMU references its Address Translation Cache (ATC) and drives out the physical address translation associated with the logical access. In parallel, the PMMU also performs checks on protection information stored along with the physical address, and asserts control signals for either a normal continuation or exceptional termina- tion of the bus cycle. If, however, there is no entry in the PMMU ATC which matches the logical access, the PMMU must perform a tablewalk to fetch the translation and protection information from its tree-like transla- tion tables.

   The PMMU uses what is known as a "root pointer" to determine where to initiate its tablewalk. The root pointer points to the base address (root) of the translation tree used for a task. Different translation mappings and protection information for tasks are contained in separate translation trees. When a context (task) switch occurs, the user root pointer value is updated to point at the root of the new tasks translation tree.

   Each DMA task may also have its own translation table. Since DMA activity may overlap the execution time of more than one user task, pointers to the DMA translation tables have to be included...