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Clarify "same pointers"
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Diomidis Spinellis
Diomidis Spinellis
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The 8086 is a 16-bit processor. One possibility for implementing some form of process isolation is to use the processor's segment registers (CS, DS, SS, ES). These allow a process's stack (SS), heap (DS, ES), and code (CS) reside in specific 64kB areas of a 1MB address space. This works by left shifting the 16-bit segment register by four bits and adding to that the 16-bit stack pointer (SS << 4 + SP), instruction pointer (CS << 4 + IP), or data address (e.g. CS << 4 + SI), to obtain the 20 bits of the physical address.

Thus, through a suitable segment register setup one can isolate a process to at most 64kB, provided the process follows the convention of not altering the segment registers. For the requirements of C programs, where the heap and the stack must be addressable through the same 16-bit pointers, this convention restricts them to 64kB of data and 64kB of code. Although this might sound overly restrictive, remember that early Unix run on a PDP-11 with 64kB of RAM. Consequently, providing a 1MB memory for multiple processes with up to 64kB of code and 64kB of data is more than generous.

Furthermore, by manipulating segment registers and copying memory regions, a supervisor program can dynamically readjust memory regions as processes are created and destroyed in a way that's transparent to running processes. Early versions of Andrew Tannembaum's MINIX operating system relied on some of these ideas.

The 8086 is a 16-bit processor. One possibility for implementing some form of process isolation is to use the processor's segment registers (CS, DS, SS, ES). These allow a process's stack (SS), heap (DS, ES), and code (CS) reside in specific 64kB areas of a 1MB address space. This works by left shifting the 16-bit segment register by four bits and adding to that the 16-bit stack pointer (SS << 4 + SP), instruction pointer (CS << 4 + IP), or data address (e.g. CS << 4 + SI), to obtain the 20 bits of the physical address.

Thus, through a suitable segment register setup one can isolate a process to at most 64kB, provided the process follows the convention of not altering the segment registers. For the requirements of C programs, where the heap and the stack must be addressable through the same pointers, this convention restricts them to 64kB of data and 64kB of code. Although this might sound overly restrictive, remember that early Unix run on a PDP-11 with 64kB of RAM. Consequently, providing a 1MB memory for multiple processes with up to 64kB of code and 64kB of data is more than generous.

Furthermore, by manipulating segment registers and copying memory regions, a supervisor program can dynamically readjust memory regions as processes are created and destroyed in a way that's transparent to running processes. Early versions of Andrew Tannembaum's MINIX operating system relied on some of these ideas.

The 8086 is a 16-bit processor. One possibility for implementing some form of process isolation is to use the processor's segment registers (CS, DS, SS, ES). These allow a process's stack (SS), heap (DS, ES), and code (CS) reside in specific 64kB areas of a 1MB address space. This works by left shifting the 16-bit segment register by four bits and adding to that the 16-bit stack pointer (SS << 4 + SP), instruction pointer (CS << 4 + IP), or data address (e.g. CS << 4 + SI), to obtain the 20 bits of the physical address.

Thus, through a suitable segment register setup one can isolate a process to at most 64kB, provided the process follows the convention of not altering the segment registers. For the requirements of C programs, where the heap and the stack must be addressable through the same 16-bit pointers, this convention restricts them to 64kB of data and 64kB of code. Although this might sound overly restrictive, remember that early Unix run on a PDP-11 with 64kB of RAM. Consequently, providing a 1MB memory for multiple processes with up to 64kB of code and 64kB of data is more than generous.

Furthermore, by manipulating segment registers and copying memory regions, a supervisor program can dynamically readjust memory regions as processes are created and destroyed in a way that's transparent to running processes. Early versions of Andrew Tannembaum's MINIX operating system relied on some of these ideas.

Source Link
Diomidis Spinellis
Diomidis Spinellis
  • 141
  • 6

The 8086 is a 16-bit processor. One possibility for implementing some form of process isolation is to use the processor's segment registers (CS, DS, SS, ES). These allow a process's stack (SS), heap (DS, ES), and code (CS) reside in specific 64kB areas of a 1MB address space. This works by left shifting the 16-bit segment register by four bits and adding to that the 16-bit stack pointer (SS << 4 + SP), instruction pointer (CS << 4 + IP), or data address (e.g. CS << 4 + SI), to obtain the 20 bits of the physical address.

Thus, through a suitable segment register setup one can isolate a process to at most 64kB, provided the process follows the convention of not altering the segment registers. For the requirements of C programs, where the heap and the stack must be addressable through the same pointers, this convention restricts them to 64kB of data and 64kB of code. Although this might sound overly restrictive, remember that early Unix run on a PDP-11 with 64kB of RAM. Consequently, providing a 1MB memory for multiple processes with up to 64kB of code and 64kB of data is more than generous.

Furthermore, by manipulating segment registers and copying memory regions, a supervisor program can dynamically readjust memory regions as processes are created and destroyed in a way that's transparent to running processes. Early versions of Andrew Tannembaum's MINIX operating system relied on some of these ideas.