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The initial address for the DTA was set to overlay the area in the PSP (at address 0x80) where the command line arguments were stored, such that a program needed to parse this area for command line arguments before invoking DOS functions that made use of the DTA (such as reading in a file record), unless the program took care to change the ...
In computing, protected mode, also called protected virtual address mode, [1] is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as segmentation, virtual memory, paging and safe multi-tasking designed to increase an operating system's control over application software.
All other references to data use the data segment. The extra segment is the default destination for string operations (for example MOVS or CMPS). FS and GS have no hardware-assigned uses. The instruction format allows an optional segment prefix byte which can be used to override the default segment for selected instructions if desired. [2]
When the CPU cache was moved inside the CPU, the CPUs implemented fixed-range MTRRs which cover the first megabyte of memory to be compatible to what PC-BIOSes provided at that time. These are used to control the cache policy needed for VGA accesses and all other memory-accesses done while the system is in real mode .
The stack segment register (SS) is usually used to store information about the memory segment that stores the call stack of currently executed program. SP points to current stack top. By default, the stack grows downward in memory, so newer values are placed at lower memory addresses.
Segmentation refers to dividing a computer's memory into segments. A reference to a memory location includes a value that identifies a segment and an offset within that segment. A segment descriptor may limit access rights, e.g., read only, only from certain rings.
The Global Descriptor Table (GDT) is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, including the base address, the size, and access privileges like executability and writability.
Primary segment-table length 2 1-25 Dispatchable-unit-control-table origin 3 0-15 PSW-key mask 3 16-31 Secondary ASN 4 0-15 Authorization index 4 16-31 Primary ASN 5 0 Subsystem-linkage control 5 1-24 Linkage-table origin 5 25-31 Linkage-table length 5 1-25 When the address-space-function control is one, Primary-ASN-second-table-entry 6 0-7