MEM

Section: Devices and Network Interfaces (4)
Updated: tahoe
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BSD mandoc
 

NAME

mem kmem vmem - main memory  

DESCRIPTION

The special file /dev/mem is an interface to the physical memory of the computer. Byte offsets in this file are interpreted as physical memory addresses. Reading and writing this file is equivalent to reading and writing memory itself. Only offsets within the bounds of /dev/mem are allowed.

Kernel virtual memory is accessed through the interface /dev/kmem in the same manner as /dev/mem Only kernel virtual addresses that are currently mapped to memory are allowed.

The file /dev/vmem allows access to the mapped portion of the VERSAbus I/O space. On the Tahoe the upper megabyte of the physical address space is accessible through this file (0xfff00000 through 0xffffffff). Accesses to the upper 64 kilobytes of the I/O space result in VERSAbus transfers with a 16-bit address (the offset in this region) and a ``non-privileged short I/O '' VERSAbus address modifier. Accesses to the remainder of the mapped region, result in VERSAbus transfers with a 24-bit address and a ``non-privileged standard'' VERSAbus address modifier. This region is actually part of the region between 0xfeff0000 and 0xffff0000 which generates VERSAbus transfers with a 24-bit address. Accesses to the remainder of the one gigabyte I/O space generate transfers that utilize a 32-bit address with a ``non-privileged extended'' address modifier. Any 32-bit address generated by a cpu access to this part of the I/O space have the upper two bits zero; thus, for example, an access to physical address 0xfe000000 causes the address 0x3e000000 to be supplied in the resultant VERSAbus read/write cycle.

On the Tahoe, the base address for the per-process data of the current process is virtual address 0xbffff000.  

FILES

/dev/mem
/dev/kmem
/dev/vmm

 

HISTORY

The mem kmem files appeared in AT&T System v6 . The file vmem appeared in BSD 4.3 tahoe  

BUGS

Memory files are accessed one byte at a time, an inappropiate method for some device registers.


 

Index

NAME
DESCRIPTION
FILES
HISTORY
BUGS

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Time: 06:48:29 GMT, May 19, 2025