BCM2835 データシートの表示（PDF） - Broadcom Corporation

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BCM2835

BCM2835 ARM Peripherals

Broadcom Corporation 

Addresses in ARM Linux are:

• issued as virtual addresses by the ARM core, then

• mapped into a physical address by the ARM MMU, then

• mapped into a bus address by the ARM mapping MMU, and finally

• used to select the appropriate peripheral or location in RAM.

1.2.2 ARM virtual addresses (standard Linux kernel only)

As is standard practice, the standard BCM2835 Linux kernel provides a contiguous mapping

over the whole of available RAM at the top of memory. The kernel is configured for a

1GB/3GB split between kernel and user-space memory.

The split between ARM and GPU memory is selected by installing one of the supplied

start*.elf files as start.elf in the FAT32 boot partition of the SD card. The minimum amount

of memory which can be given to the GPU is 32MB, but that will restrict the multimedia

performance; for example, 32MB does not provide enough buffering for the GPU to do

1080p30 video decoding.

Virtual addresses in kernel mode will range between 0xC0000000 and 0xEFFFFFFF.

Virtual addresses in user mode (i.e. seen by processes running in ARM Linux) will range

between 0x00000000 and 0xBFFFFFFF.

Peripherals (at physical address 0x20000000 on) are mapped into the kernel virtual address

space starting at address 0xF2000000. Thus a peripheral advertised here at bus address

0x7Ennnnnn is available in the ARM kenel at virtual address 0xF2nnnnnn.

1.2.3 ARM physical addresses

Physical addresses start at 0x00000000 for RAM.

• The ARM section of the RAM starts at 0x00000000.

• The VideoCore section of the RAM is mapped in only if the system is configured to

support a memory mapped display (this is the common case).

The VideoCore MMU maps the ARM physical address space to the bus address space seen

by VideoCore (and VideoCore peripherals). The bus addresses for RAM are set up to map

onto the uncached1 bus address range on the VideoCore starting at 0xC0000000.

Physical addresses range from 0x20000000 to 0x20FFFFFF for peripherals. The bus

addresses for peripherals are set up to map onto the peripheral bus address range starting at

0x7E000000. Thus a peripheral advertised here at bus address 0x7Ennnnnn is available at

physical address 0x20nnnnnn.

1.2.4 Bus addresses

The peripheral addresses specified in this document are bus addresses. Software directly

accessing peripherals must translate these addresses into physical or virtual addresses, as

described above. Software accessing peripherals using the DMA engines must use bus

addresses.

1 BCM2835 provides a 128KB system L2 cache, which is used primarily by the GPU. Accesses to memory are

routed either via or around the L2 cache depending on senior two bits of the bus address.

06 February 2012 Broadcom Europe Ltd. 406 Science Park Milton Road Cambridge CB4 0WW

© 2012 Broadcom Corporation. All rights reserved

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