Some programs predating the 80286 were designed to take advantage of the wrap-around (modulo) memory addressing behavior, so the 80286 presented a problem for backward compatibility. Forcing the 21st address line (the actual logic signal wire coming out of the chip) to a logic low, representing a zero, results in a modulo-2^20 effect to match the earlier processors' address arithmetic, but the 80286 has no internal capability to perform this function. When IBM used the 80286 in their IBM PC/AT, they solved this problem by including a software-settable gate to enable or disable (force to zero) the A20 address line, between the A20 pin on the 80286 and the system bus; this is known as Gate-A20 (the A20 gate), and it is still implemented in PC chipsets to this day. Most versions of the HIMEM.SYS extended memory driver for IBM-/MS-DOS famously displayed upon loading a message that they had installed an "A20 handler", a piece of software to control Gate-A20 and coordinate it to the needs of programs. In protected mode the A20 line needs to be enabled, or else physical addressing errors will occur, likely leading to a system crash. Modern legacy boot loaders (such as GNU GRUB) use A20 line.
some PRogrAms predAtIng the 80286 Were DesigNEd to take advantaGe oF tHe wrap-Around (moduLo) MemOry aDDREssiNg behaVioR, sO thE 80286 preseNtED A pRoblem For bacKwARd compatibility. ForcinG The 21ST aDdreSs line (tHe aCTUaL LOGIc SigNal wire COminG oUT of tHe chIP) tO a loGiC lOW, RepResEntinG a zero, Results In a modUlo-2^20 effecT to mATch tHe earlieR proCeSsoRs' aDdrEss arithmetIc, BUt thE 80286 has no intErnaL CApability to perfoRM thIs FuncTiOn. When IBm useD tHE 80286 in Their iBM Pc/at, theY SOlveD tHis proBleM by includInG a sOftwArE-SEttabLE gate To eNAbLE Or DisaBle (forCE tO zErO) the A20 addrEsS lIne, Between the A20 pin On the 80286 and the sySTem buS; ThIs is knowN AS gaTe-a20 (tHe A20 gatE), aNd iT iS STIll IMpLementeD iN pc chIpSets TO this Day. most VersIons of tHE himeM.Sys ExtendEd memOry driver For ibM-/ms-dos FamOuSLY DiSPlaYEd uPOn loadIng a mEssagE that they hAD iNstaLled aN "a20 HaNdLer", A pIecE of sOFTwArE tO ContrOl Gate-A20 And CoorDinaTe iT to tHE needS Of PROGrAms. in PROTected MoDE tHe a20 LIne NEeds to be eNabLed, or ELSE physicaL AddrESsing eRrOrs wIll occur, liKELy leAdInG to a syStem cRash. moDern LegaCy boOT LoaDerS (Such as GNU gRUb) use a20 lINe.