MEGA65 Welcome Guide
4.3 ROMs
While the core recreates the hardware of the Commodore 65, the ROM recreates the built-in software of the Commodore
65. This software interfaces with the hardware and provides built-in functionality like BASIC programming.
The original Commodore 65 kept its software on a physical Read-Only Memory (ROM) chip. With MEGA65, this
software lives in a file on the SD card named MEGA65.ROM. MEGA65 loads this into memory when you turn it on. This
file contains the original software from the C65 ROM chip licensed from the copyright holder, with many fixes and
improvements. You can upgrade the operating system by replacing this file on the SD card.
Note: Newer versions of the MEGA65 ROM require a newer version of the MEGA65 core. Following this Guide, you
will update both the ROM and the core together.
Tip: The User Guide describes ROM files starting on page 26.
4.4 The Hypervisor
The MEGA65 has additional built-in functionality not present in the original Commodore 65, such as for managing
the startup sequence, SD cards, cores, and configuration. This operating system is known as the Hypervisor.
You see the Hypervisor in action every time you turn on your MEGA65. Normally this screen displays some messages
then quickly disappears. If you want to pause the Hypervisor to read these messages, hold the Ctrl key during startup.
Release Ctrl to continue.
4.5 PAL and NTSC
Back in the day, PAL and NTSC were competing standards for analog video signals, used for both transmission and
rendering on cathode ray tube (CRT) displays. They differed in two major ways:
• The number of horizontal lines used to make the image. PAL uses a vertically dense 625 interlaced lines, com-
pared to NTSC which uses 525 lines.
• The number of times the screen is drawn per second, aka the refresh rate. PAL sweeps the screen top to bottom
50 times per second (50 Hz, or 25 interlaced frames per second), while NTSC refreshes at a faster 60 times per
second (60 Hz, or 30 frames per second).
Commodore made different versions of its computers for each standard, and sold them in the countries where those
standards were used: NTSC in the United States and Japan, and PAL in Europe.
Both the raster line count and the refresh rate affect the execution of computer programs written for vintage computers
with analog video output. Software has to use precise timing to render graphics for each frame, and is written to expect
the screen to be a certain number of lines tall. Games use the refresh rate to control the timing of other events like
playing music. Without extra work by a programmer to account for the differing standards, a program written to be
compatible with one video standard might run too fast or too slow, have erratic graphical behavior, or just not work at
all on a machine built for the other standard.
The MEGA65 has an analog VGA video output and a digital HDMI video output. Both of these standards are newer
than PAL and NTSC and can support multiple resolutions and refresh rates. The MEGA65 must still be set to either
PAL mode or NTSC mode (in Configuration or the Freeze menu) to support the vintage software that might be expecting
one or the other.
4.3. ROMs 27
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