To be exact - this is not a "boot-time only" thing - if you change to tty1...6 with Ctrl+Alt+F1...F6 then the console screen has different resolution, then default one.
P.S. Sometimes btnx even worked without a problem with vga=791 (1024 x 768 resolution), but mostly not, if I recall corectly. Without this option in GRUB menu all works without problems.
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