CONFIG_KERNEL_HALF_GIGS is not available for thedefaultarchitecture x86.
Result is shown for architecture um
default or selected kernelversion does not have config value CONFIG_KERNEL_HALF_GIGS.
Result is shown for kernelversion 5.4.123

Kernel address space size (in .5G units)


Linux/Usermode Kernel Configuration
└─>UML-specific options
└─>Kernel address space size (in .5G units)
In linux kernel since version 2.6.12  
This determines the amount of address space that UML will allocate for
its own, measured in half Gigabyte units. The default is 1.
Change this only if you need to boot UML with an unusually large amount
of physical memory.