Linux内核参数“vga=”是怎样影响显示模式

若不指定“vga=”参数，系统则不支持framebuffer设备，为什么呢？在内核代码中是如何实现的？
查看系统启动日志kern.log发现，指定“vga= ”比不指定多了一下几行：

Apr 25 17:14:03 nmr107 kernel: vesafb: framebuffer at 0xd0000000, mapped to 0xe0880000, using 6144k, total 65536k

Apr 25 17:14:03 nmr107 kernel: vesafb: mode is 1024x768x32, linelength=4096, pages=1
Apr 25 17:14:03 nmr107 kernel: vesafb: protected mode interface info at c000:f6d0
Apr 25 17:14:03 nmr107 kernel: vesafb: pmi: set display start = c00cf715, set palette = c00cf79a
Apr 25 17:14:03 nmr107 kernel: vesafb: pmi: ports = b4c3 b503 ba03 c003 c103 c403 c503 c603 c703 c803 c903 cc03 ce03 cf03 d003 d103 d203 d303 d403 d503 da03 ff03
Apr 25 17:14:03 nmr107 kernel: vesafb: scrolling: redraw
Apr 25 17:14:03 nmr107 kernel: vesafb: Truecolor: size=8:8:8:8, shift=24:16:8:0
可以看出这里设置了vesafb模式