Quite surprising for me as a long time Atari 65XE user is that those PRG were starting with a basic command. On Atari all binary programs were loaded without BASIC in memory. If you forgot to disable basic, there was a chance the program wouldn't run or would hang. I guess this must have been due to different memory layout?

The C64 starts up straight into BASIC from ROM. Unlike some other contemporary computers, it doesn't attempt to boot from any external devices (except the cartridge port). There isn't really a DOS in the usual sense. Apart from simple support for loading and saving programs, and a very basic channel I/O facility, everything else is handled by the firmware in the disk drive, which has its own 6502 and operating system.

For example, there's no command for getting a directory listing. You type `LOAD "$",8` (8 being the disk drive), and the drive pretends there's a BASIC program called `$` that happens to contain a directory listing you can then look at with `LIST`. (https://en.wikipedia.org/wiki/Commodore_DOS#/media/File:Comm...)

By default, LOAD loads tokenized BASIC programs, but if you add an extra `,1` to the command, the file can contain arbitrary data starting at any location in memory. You could use this to load a machine language program and then run it with `SYS <location>`. Clever programmers figured out they could skip this step by having their file overwrite a vector that gets called after the load completes and jump right into their code, resulting in every Commodore kid having being able to type `LOAD"*",8,1` on autopilot.

I got distracted by other trivia (I grew up with this computer and it was hugely influential and I will adore it forever) from getting to the actual point: The C64 uses a variant of the 6502, the 6510. It has a special register for swapping out any combination of the three ROMs (BASIC, KERNAL (sic), and the character ROM) plus I/O registers that overlay portions of the 64 address space. If your code doesn't use those, you can access the RAM they are hiding by turning them off.

The C64 always booted to BASIC, specifically CBM BASIC 2.0. There wasn't a provision for automatic booting from disk until at least the C128.

LOAD "*",8,1 was the command to load the first file off of your attached 1541 (if you were lucky enough to have multiple 1541s, your first one would be device 8 and you'd have had to set the device number on others to 9 or higher). Anyone who had and played a lot of games on the C64 back in the day has this command etched in their permanent memory.

There was the convenient-looking RUN/STOP key (yes it is confusing, it's STOP without SHIFT, and RUN with SHIFT held down) but the RUN key would only auto-load from device 1 which was the cassette. Made sense in 1982 when the machine was released because disk drives were about $500 in 1982 dollars, the same price as the system itself.

BASIC 2.0 had no "BLOAD" or "BRUN" to directly load and/or run a binary executable. The underlying Kernal could do this, but BASIC left a LOT of functionality on the C64 unexposed (such as - all the sprites and graphics modes). So the standard was to release programs that were in a form that would look like a BASIC program.

So C64 BASIC doesn't have a BLOAD command but ... it kinda did in a janky way. The ,1 in LOAD"*",8,1 means to "not relocate the file" - and any PRG file on a 1541 disk will have the intended address of the "program" as its first two bytes. If the ,1 is present, BASIC will tell the Kernal to load the file at the address it specifies. (There was no SAVE"xxx",8,1). Some software would load to an address that would overwrite the BASIC main loop vectors and immediately start without having to type RUN afterward. Without the ,1 BASIC will load it into the BASIC program text space at 2048.

Much other software was a hacked BASIC program that had one command, something like 10 SYS 2057 or similar, and then 6502 code right after the necessary codes that told BASIC the end of program text was reached. BASIC program text started at memory location 2048, right after the 1K of screen RAM at 1024. SYS is a command that simply jumps to a machine language program - and in this case, it's jumping to the binary tacked on to the end of the small BASIC program, which would be the game or at least a booting program for the game.

Programs like this had the actual address in the PRG matching what BASIC would want, so LOAD "*",8 or LOAD"*",8,1 typically made no difference unless the game was that auto-RUNing type.

The C64 had 4K of RAM not used for anything else at 49152. It was common for utilities to live there, so you'd load those in with a LOAD"something",8,1 and then SYS 49152 to start them.

On the Atari's you could also run 6502 binaries from inside Atari BASIC. The Atari ROM OS explicitly reserved page 6 of the memory map for "user use" and Atari Basic followed suit. There were (IIRC) also a tiny number of page 0 bytes reserved for 'user use' as well.

So, as long as your entire binary fit into 256 bytes, you could run it from inside BASIC. In fact, you could even store it as a BASIC program, the BASIC just needed to "POKE" the binary into page 6, and then you could jump to it.

To do anything larger than 256 bytes required you to dig into the inner workings of where BASIC stored code itself and avoid overwriting any of BASIC's data, or having it overwrite any of your code. Not impossible to do, but did require a lot of undocumented (or not so well documented) work.

IIRC the C64, like the BBC range of 6502-based Micros, had their BASIC in ROM and in fact booted to it in REPL mode by default. As such it was always in memory as the ROM would always be there¹. There were certain bits of the address space that were not safe to use if the user would drop back into BASIC as the ROM used those for its stack & heap and other scratch space, but otherwise you could ignore BASIC's existence once your machine code was running.

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[1] caveat: under usual circumstances, on the Beebs you could switch a different bank into the memory space used by the BASIC ROM, either another ROM or some “sideways RAM”

C64 always had basic built in. Because it was there you could assume it, and further it being there affected how the system booted. I'm was never a C64 guy so don't know what the options were for someone who didn't want basic.

The original Atari didn't have built in basic (the 2nd generation XL and your 3rd generation XE series both did). As such Atari programmers could never assume basic, and even when you could assume basic you couldn't assume the version, there were a few bugs in various versions. (I knew someone with a XL who used the earlier version of basic because the bugs in the XL version were serious enough to affect his usage).