A20 line
That was accomplished by inserting a logic gate on the A20 line between the processor and system bus, which got named Gate-A20.
Enabling the Gate-A20 line is one of the first steps that a protected-mode x86 operating system does in the bootup process, often before control has been passed to the kernel from the bootstrap (in the case of Linux, for example).
In what was arguably a "hack", the A20 gate was originally part of the keyboard controller on the motherboard, which could open or close it depending on what behavior was desired.
Such operating systems had no compatibility reasons for keeping it closed, and they gained access to the full range of physical addresses available by opening it.
The Intel 80486 and Pentium added a special pin named A20M#, which when asserted low forces bit 20 of the physical address to be zero for all on-chip cache- or external-memory accesses.
[6] The PC System Design Guide PC 2001 removes compatibility for the A20 line: "If A20M# generation logic is still present in the system, this logic must be terminated such that software writes to I/O port 92, bit 1, do not result in A20M# being asserted to the processor.
[citation needed] From a software point of view, the mechanism works exactly as before, and an operating system must still program external hardware (which in-turn sends the aforementioned bus cycles to the CPU) to disable the A20 masking.
Since 1980, the address wrap was internally used by 86-DOS and MS-DOS to implement the DOS CALL 5 entry point at offset +5 to +9 (which emulates the CP/M-80-style CALL 5 BDOS API entry point at offset +5 to +7) in the Program Segment Prefix (PSP) (which partially resembles CP/M-80's zero page).
[9][10] This was, in particular, utilized by programs machine-translated from CP/M-80 through assembly language translators[9] like Seattle Computer Products' TRANS86.
[11] The CALL 5 handler this entry point refers to resides at the machine's physical address 0x000000C0 (thereby overlapping the four bytes of the interrupt service routine entry point reserved for INT 30h and the first byte of INT 31h in the x86 real mode interrupt vector table).
[13][14] The only way to reconcile these requirements was to choose a segment value that, when added to 0xFEF0, results in an address of 0x001000C0, which, on an 8086, wraps around to 0x000000C0.
[17] Also, the SPELL utility in Microsoft's Word 3.0 (1987) is one of the programs depending on the CALL 5 interface to be set up correspondingly.
This trick works for as long as the code isn't executed in low memory, the first 64 KB of RAM, a condition that was always true in older DOS versions without load-high capabilities.
[17] Other commonly used development utilities using this were executable compressors like Realia's Spacemaker[20] (written by Robert B. K. Dewar in 1982 and used to compress early versions of the Norton Utilities[24][25][26][27]) and Microsoft's EXEPACK[19][20][1][28][17] (written by Reuben Borman in 1985) as well as the equivalent /E[XEPACK] option in Microsoft's LINK 3.02 and higher.
[19][1][28][26] Programs processed with EXEPACK would display a "Packed file is corrupt" error message.
[1][20][28] Various third-party utilities exist to modify compressed executables either replacing the problematic uncompression routine(s) through restubbing, or attempting to expand and restore the original file.
