Editing Commodore Plus/4 (section)

== Performance ==

While the Plus/4's CPU could run about 75% faster than the C64's, the computer was still designed with a [[shared graphics memory|shared memory]] architecture, in which screen data resided in main memory. This means that the video chip has to access the memory while it is displaying the picture, in effect slowing down the CPU to less than half its full speed in this screen area. Only during those periods when the video chip is displaying the screen border or putting out the horizontal and vertical retrace signals is the CPU able to run at full speed. This means that, on average, with standard 40x25 screen the Plus/4's CPU runs only about 15% faster than that of the C64. However, as mentioned the Plus/4 can be 75% faster than a C64 when using screen blanking mode. The Plus/4's [[PAL]] model may also be switched to [[NTSC]] mode by disabling the screen which sets the CPU frequency to 2.22&nbsp;MHz<ref>{{cite web|url=http://www.lemon64.com/forum/viewtopic.php?t=44377|title=6502 actual speed at lemon64|date=30 September 2012 }}</ref> (this is 115% faster than C64's maximum speed). So a Plus/4 (PAL) is one of the fastest 6502 based computers for raw calculations. This is true for programs in [[machine code]] but BASIC 3.5 is slightly slower than BASIC 2.0 due to its greater size, and performance slows down further when programs cross over the {{mono|$8000}} line because the BASIC ROM has to be switched out to read BASIC program text. The NTSC model of Plus/4 is about 10% slower than the PAL model displaying a standard screen but slightly faster with screen blank.