en.wikipedia.org/wiki/Real_mode
1 correction found
64-bit operating systems use real mode only at the startup stage, after which the OS kernel will switch the CPU into long mode.
This is too broad: many 64-bit x86 operating systems boot via UEFI and are entered in 64-bit long mode, not real mode.
Full reasoning
This sentence is incorrect as a blanket statement about 64-bit operating systems. On x64 UEFI systems, firmware hands off control in long mode, in 64-bit mode, so a 64-bit OS or boot loader does not have to begin in real mode.
The UEFI specification's x64 platform section says that during boot services time the processor is in "Long mode, in 64-bit mode" with paging enabled. Linux's x86 boot protocol likewise documents a 64-bit entry path where "At entry, the CPU must be in 64-bit mode with paging enabled."
So while legacy BIOS boot paths often do begin in real mode, it is false to state generally that 64-bit operating systems use real mode at startup and only later switch into long mode.
2 sources
- 2. Overview — UEFI Specification 2.10 documentation
When a 32-bit UEFI OS is loaded, the system firmware hands off control to the OS in flat 32-bit mode. ... 2.3.4. x64 Platforms ... Long mode, in 64-bit mode ... Paging mode is enabled and any memory space defined by the UEFI memory map is identity mapped.
- 1. The Linux/x86 Boot Protocol — The Linux Kernel documentation
At entry, the CPU must be in 64-bit mode with paging enabled. ... In 64-bit boot protocol, the kernel is started by jumping to the 64-bit kernel entry point, which is the start address of loaded 64-bit kernel plus 0x200.