Editing Commodore 128 (section)

== Design ==
[[file:Commodore 128 BASIC Prompt.gif|thumb|The BASIC prompt for the Commodore 128 in 40-column mode, running Commodore BASIC V7.0]]

The C128's complex architecture<ref>{{Cite web |last=Matthews |first=Ian |date=2003-07-11 |title=The Commodore 128: The Most Versatile 8-Bit Computer Ever Made |url=http://www.commodore.ca/products/128/Commodore_128.htm |archive-url=https://web.archive.org/web/20030726185922/http://www.commodore.ca/products/128/Commodore_128.htm |archive-date=2003-07-26 |access-date=2020-12-28 |website=commodore.ca}}</ref><ref>{{Cite web |last=Bennett |first=Chris |title=New Computers At The CES Show |url=http://www.commodore.ca/gallery/announcements/128_lcd_announcement_TPUG_Feb_1985.pdf |url-status=dead |archive-url=https://web.archive.org/web/20121022111843/http://www.commodore.ca/gallery/announcements/128_lcd_announcement_TPUG_Feb_1985.pdf |archive-date=22 October 2012 |access-date=13 July 2011}}</ref> includes four differently accessed kinds of RAM (128&nbsp;KB main RAM, 16–64&nbsp;KB VDC video RAM, 2&nbsp;kNibbles VIC-II Color RAM, 2-KB floppy-drive RAM on C128Ds, 0, 128 or 512&nbsp;KB [[Commodore REU|REU]] RAM){{failed verification|date=March 2023}}, two or three CPUs (main: 8502, Z80 for CP/M; the 128D also incorporates a 6502 in the disk drive), and two different video chips (VIC-IIe and VDC) for its various operational modes.<ref>{{Cite web |date=2016-05-08 |title=Commodore 128D computer |url=http://oldcomputers.net/c128d.html |access-date=2020-12-28 |website=www.oldcomputers.net}}</ref><ref>{{Cite web |url=http://www.datasalen.se/Utstallning/Data/CBM/commodore128deng.htm |title=Commodore 128D |access-date=16 February 2011 |archive-url=https://web.archive.org/web/20110724065210/http://www.datasalen.se/Utstallning/Data/CBM/commodore128deng.htm |archive-date=24 July 2011 |url-status=dead }}</ref>

The C128 does not perform a system RAM test on power-up like previous Commodore machines.{{citation needed|date=December 2023}} Instead of the single [[MOS Technology 6510|6510]] [[microprocessor]] of the C64, the C128 incorporates a two-CPU design. The primary CPU, the [[MOS Technology 8502|8502]], is a slightly improved version of the 6510, capable of being clocked at 2&nbsp;[[megahertz|MHz]]. The second CPU is a Zilog Z80 which is used to run CP/M software, as well as to initiate operating-mode selection at boot time.

The C128's keyboard includes four [[Arrow keys|cursor keys]], {{keypress|Alt}}, {{keypress|Help}}, {{keypress|Esc}} and {{keypress|Tab}} keys and a numeric keypad. None of these were present on the C64 which had only two cursor keys, requiring the use of the {{keypress|Shift}} key to move the cursor up or left. This alternate arrangement was retained on the 128, for use under C64 mode.<ref>{{Cite web|url=http://www.commodore.ca/manuals/128_system_guide/sect-09.htm|title=C128 System Guide – 9.5 MOVING THE CURSOR IN C64 MODE|website=commodore.ca|access-date=8 August 2016}}</ref> The lack of a numeric keypad, {{keypress|Alt}} , and {{keypress|Esc}} on the C64 was an issue with some CP/M [[productivity software]] when used with the C64's Z80 cartridge. A keypad was requested by many C64 owners who spent long hours entering [[machine language]] [[type-in program]]s.<ref>{{cite book|quote=A numeric keypad (the number keys clustered together as they are on a calculator) is useful for anyone entering large amounts of numeric data, but this feature was lacking on the Commodore 64. Since many Commodore 64 owners spent hours entering machine language programs in the form of long lists of numbers, this feature was often asked for.|title=The Official Book for the Commodore 128|year=1982|isbn=9780672224560|url=https://archive.org/details/The_Official_Book_for_the_Commodore_128/mode/2up|last1=Waite|first1=Mitchell|last2=Lafore|first2=Robert W.|last3=Volpe|first3=Jerry|publisher=H.W. Sams }}</ref> Many of the added keys matched counterparts present on the [[IBM PC compatible|IBM PC]]'s keyboard and made the new computer more attractive to business software developers.<ref>{{harvnb|Waite|Lafore|Volpe|1982}}: "...they make the C128 more attractive to those established business software developers who are used to working with more traditional keyboard arrangements."</ref> While the 128's 40-column mode closely duplicates that of the C64, an extra 1K of color RAM is made available to the programmer, as it is multiplexed through memory address 1.{{citation needed|date=December 2023}}

The design of the C128's power supply is different from that used with the C64.  Although it is much larger, the new power supply is equipped with cooling vents and a replaceable fuse.{{citation needed|date=December 2023}}

Early versions of the C128 occasionally experience temperature-related reliability issues due to the use of an [[RF shield]] over the main circuit board. The shield was equipped with fingers that contacted the tops of the major chips, ostensibly causing the shield to act as a large [[heat sink]]. A combination of poor contact between the shield and the chips, the inherently limited heat conductivity of plastic chip packages, as well as the relatively poor thermal conductivity of the shield itself, resulted in overheating and failure in some cases. The [[MOS Technology 6581|SID]] sound chip is particularly vulnerable in this respect. The most common remedy is to remove the shield, which Commodore had added late in development to comply with [[Federal Communications Commission|FCC]] radio-frequency regulations.{{original research inline|date=December 2023}}

The C128 has three [[Mode (user interface)|operating modes]]. ''C128 Mode'' ([[Native (computing)|native mode]]) runs at 1 or 2&nbsp;MHz with the 8502 CPU and has both 40- and 80-column [[text mode]]s available. ''CP/M Mode'' uses both the Z80 and the 8502,<ref>Commodore CP/M Source code and programmers' manual.</ref> and is able to function in both 40- and 80-column text mode. ''C64 Mode'' is nearly 100 percent compatible with the earlier computer. Selection of these modes is implemented via the Z80 chip. The Z80 controls the bus on initial boot-up and checks to see if there is a CP/M disk in the drive, if there are any C64/C128 cartridges present, or if the Commodore key (which serves as the C64-mode selector) is being depressed on boot-up. Based on these conditions, it will switch to the appropriate mode of operation.{{citation needed|date=December 2023}}