System Software 1.0 (System 3.1/Finder 5.2, Feb. 1986) for the 512K is presumably not what you wish to know about, given your usual research instincts, so I was driven back to Cary Lu's book for the authorship of System 1.0.
Jobs (I still have difficulty in assimilating my recent discovery that he is pronounced as 'tasks' or 'occupations' rather than as the Biblical name) and Gates are recorded by Lu, a Microsoft employe writing a book for Microsoft Press,
The Apple Macintosh Book, as having consulted closely during the hardware and software development for the Macintosh. Certainly this embraced applications, but I cannot corroborate my long-held remembrance that Gates,
inter alia perhaps, had a hand in coding the System.
What Lu's book (1984) did reveal was this:
... the only way to add more memory without redesigning is to use denser memory chips. The initial Mac uses 64-kilobit (K) RAM chips. As 256-kilobit RAM chips become widely available at a reasonable cost, they will be installed in the Macintosh, quadrupling the total memory to 512kB.
The 256-k chips are the same size as the 64-k chips, so there is no need to redesign the board. A data-selector component, two resistors, and a capacitor must be changed as well. All the software and the ROM can handle the 512-kB memory already.
The added emphasis is mine.
Lu goes on to hint at, but not forecast, future developments with the statement that the MC68000 can directly address up to 16MB of RAM.
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