Editing Bounds checking (section)

== Hardware bounds checking ==

The safety added by bounds checking necessarily costs CPU time if the checking is performed in software; however, if the checks could be performed by hardware, then the safety can be provided "for free" with no runtime cost. An early system with hardware bounds checking was the [[ICL 2900 Series]] mainframe announced in 1974.<ref>{{cite book |pages=17, 77 |url=http://www.fujitsu.com/uk/Images/icl-2900-series-by-jk-buckle.pdf |title=The ICL 2900 Series |author=J. K. Buckle |publisher=Macmillan Computer Science Series |year=1978 |isbn=978-0-333-21917-1 |access-date=20 April 2018 |archive-date=20 April 2018 |archive-url=https://web.archive.org/web/20180420203006/http://www.fujitsu.com/uk/Images/icl-2900-series-by-jk-buckle.pdf |url-status=dead }}</ref> The [[VAX]] computer has an INDEX assembly instruction for array index checking which takes six operands, all of which can use any VAX addressing mode. The B6500 and similar [[Burroughs Corporation|Burroughs]] computers performed bound checking via hardware, irrespective of which computer language had been compiled to produce the machine code.  A limited number of later [[CPU]]s have specialised instructions for checking bounds, e.g., the CHK2 instruction on the [[Motorola 68000#Interrupts|Motorola 68000]] series.

Research has been underway since at least 2005 regarding methods to use x86's built-in virtual memory management unit to ensure safety of array and buffer accesses.<ref>{{Cite book | doi=10.1109/DSN.2005.25| chapter=Checking Array Bound Violation Using Segmentation Hardware| title=2005 International Conference on Dependable Systems and Networks (DSN'05)| pages=388–397| year=2005| last1=Lap-Chung Lam| last2=Tzi-Cker Chiueh| isbn=0-7695-2282-3| s2cid=6278708}}</ref> In 2015 Intel provided their [[Intel MPX]] extensions in their [[Skylake (microarchitecture)|Skylake]] processor architecture which stores bounds in a CPU register and table in memory. As of early 2017 at least [[GNU Compiler Collection|GCC]] supports MPX extensions.