Although it's the source code for the Atari 800 version of PCS, it seems to have been built on an Apple II. From MAKE1 [1] for example: CALL -151 3F2:

Although it's the source code for the Atari 800 version of PCS, it seems to have been built on an Apple II. From MAKE1 [1] for example: CALL -151 3F2:69 FF 5A BLOAD GPAK.OBJ,A$1300 BLOAD CDRAW.O,A$2480 BLOAD PCSA800.PIC,A$2B00 BLOAD DLIST.O,A$4910 BLOAD SWAP.O,D2,A$4A00 BLOAD TST.O,D1,A$4B00 BLOAD GOATARI.O,D2,A$4B45 BLOAD BOOT.O,D2,A$8E00 BLOAD BOOT2.O,A$8E80 BLOAD ZAP1.O,A$8F00 BLOAD DOWNLOAD.O,D1,A$300 The first line invokes the Apple II "System Monitor", the second writes 3 bytes to memory at address $3F2 (the effect of which is to tell the computer to enter the Monitor whenever someone resets the computer from the keyboard), and the rest load binary files into different memory areas. I'm guessing it's all to build an image to be written to disk. (See [2] for more on the System Monitor.) [1] https://github.com/billbudge/PCS_Atari800/blob/master/Disk1/MAKE1 https://github.com/billbudge/PCS_Atari800/blob/master/Disk1/... [2] http://apple2.info/wiki/index.php?title=System_Monitor#General_Intro_to_this_reference_and_System_Monitor http://apple2.info/wiki/index.php?title=System_Monitor#Gener... EDIT: added additional details