Hardware Reference
In-Depth Information
technique, the register always points to the physical address of the start of the cur-
rent program. All memory addresses have the contents of the relocation register
added to them by the hardware before being sent to the memory. The entire reloca-
tion process is transparent to the user programs. They do not even know that it is
occurring. When a program is moved, the operating system must update the relo-
cation register. This mechanism is less general than paging because the entire pro-
gram must be moved as a unit (unless there are separate code and data relocation
registers, as on the Intel 8088, in which case it has to be moved as two units).
A third mechanism is possible on machines that can refer to memory relative
to the program counter. Many branch instructions are relative to the program
counter, which helps. Whenever a program is moved in main memory only the
program counter need be updated. A program, all of whose memory references are
either relative to the program counter or absolute (e.g., to I/O device registers at
absolute addresses) is said to be
position independent
. A position-independent
procedure can be placed anywhere within the virtual address space without the
need for relocation.
The linking strategy discussed in Sec. 7.4.1 has the property that all procedures
that a program might call are linked before the program can begin execution. On a
computer with virtual memory, completing all linking before beginning execution
does not take advantage of the full capabilities of the virtual memory. Many pro-
grams have procedures that are called only under unusual circumstances. For ex-
ample, compilers have procedures for compiling rarely used statements, plus pro-
cedures for handling error conditions that seldom occur.
A more flexible way to link separately compiled procedures is to link each pro-
cedure at the time it is first called. This process is known as
dynamic linking
.It
was pioneered by MULTICS whose implementation is in some ways still unsurpas-
sed. In the next sections we will look at dynamic linking in several systems.
Dynamic Linking in MULTICS
In the MULTICS form of dynamic linking, associated with each program is a
segment, called the
linkage segment
, which contains one block of information for
each procedure that might be called. This block of information starts with a word
reserved for the virtual address of the procedure and it is followed by the procedure
name, which is stored as a character string.
When dynamic linking is being used, procedure calls in the source language
are translated into instructions that indirectly address the first word of the corres-
ponding linkage block, as shown in Fig. 7-17(a). The compiler fills this word with
either an invalid address or a special bit pattern that forces a trap.
