Editing Apple II (original) (section)

==PCB revisions==
[[File:Apple II motherboard (53171103722).jpg|thumb|left|An Apple II PCB. The white rectangle outline near the middle surrounds the three RAM banks, here all loaded with 16kx1 DRAMs.]]
The Apple II's [[printed circuit board]] (PCB) underwent several revisions, as Steve Wozniak made modifications to it. The earliest version was known as Revision 0, and the first 6,000 units shipped used it. Later revisions added a "color killer" circuit to prevent color fringing when the computer was in text mode, as well as modifications to improve the reliability of cassette I/O. Revision 0 Apple IIs powered up in an undefined mode and had garbage on-screen, requiring the user to press Reset. This was eliminated in later board revisions. Revision 0 Apple IIs could display only four colors in hi-res mode, but Wozniak was able to increase this to six hi-res colors on later board revisions. (Technically it was eight, but only six were visible.<ref name="laboratorium.net">{{cite web | url=http://laboratorium.net/archive/2009/03/08/why_did_the_applie_ii_have_six_colors#comment-70109 | title=The Laboratorium : Why Did the Apple II Have Six Colors? }}</ref>)

Apple II PCBs have three [[random-access memory|RAM]] banks for a total of 24 RAM chips. Original Apple IIs had jumper switches to adjust RAM size, and RAM configurations could be 4, 8, 12, 16, 20, 24, 32, 36, or 48&nbsp;KiB. The three smallest memory configurations used 4kx1 [[Dynamic random-access memory|DRAMs]], with larger ones using 16kx1 DRAMs, or mix of 4-kilobyte and 16-kilobyte banks (the chips in any one bank have to be the same size). The early [[Apple II Plus|Apple II+]] models retained this feature, but after a drop in DRAM prices, Apple redesigned the circuit boards without the jumpers, so that only 16kx1 chips were supported. A few months later, they started shipping all machines with a full 48&nbsp;KiB complement of DRAM.

Unlike most machines, all [[integrated circuit]]s on the Apple II PCB were socketed; this cost more to manufacture and created the possibility of loose chips causing a system malfunction, but it was considered preferable to make servicing and replacement of bad chips easier.

The Apple II PCB lacks any means of generating an [[interrupt request]], although expansion cards may generate one. Program code had to stop everything to perform any [[Input/output|I/O]] task; like many of the computer's other idiosyncrasies, this was due to cost reasons and Steve Wozniak assuming interrupts were not needed for gaming or using the computer as a teaching tool.