Revision as of 04:12, 18 June 2017 view source osdev>Eladashkcenazi335 →‎Testing The A20 Line From Protected Mode ← Older edit		Revision as of 04:47, 25 October 2017 view source osdev>Slowbyte mNo edit summary Newer edit →
Line 1: The A20 Address Line is the physical representation of the 21st bit (number 20, counting from 0) of any memory access. When the IBM-AT (Intel 286) was introduced, it was able to access up to sixteen megabytes of memory (instead of the 1 MByte of the 8086). But to remain compatible with the 8086, a quirk in the 8086 architecture (memory wraparound) had to be duplicated in the AT. To achieve this, the A20 line on the address bus was disabled by default. The ~~wraparound~~workaround was caused by the fact the 8086 could only access 1 megabyte of memory, but because of the segmented memory model ~~they~~it could effectively address up to 1 megabyte and 64 kilobytes (minus 16 bytes). Because there are ~~twenty~~20 address lines on the 8086 (A0 through A19), any address above the 1 megabyte mark wraps around to zero. For some reason a few short-sighted programmers decided to write programs that actually used this wraparound (rather than directly addressing the memory at its normal location at the bottom of memory). Therefore in order to support these 8086-era programs on the new processors, this wraparound had to be emulated on the IBM AT and its compatibles; this was originally achieved by way of a latch that by default set the A20 line to zero. Later the 486 added the logic into the processor and introduced the A20M pin to control it. For an operating system developer (or [[Bootloader]] developer) this means the A20 line has to be enabled so that all memory can be accessed. This started off as a simple hack but as simpler methods were added to do it, it became harder to program code that would definitely enable it and even harder to program code that would definitely disable it.

A20 Line: Difference between revisions