From SDCC wiki
--model-flat24: This option generates code for the 24 bit contiguous addressing mode of the Dallas DS80C390 part. In this mode, up to four meg of external RAM or code space can be directly addressed. See the data sheets at www.dalsemi.com for further information on this part. Note that the compiler does not generate any code to place the processor into this mode (it defaults to 8051 compatible mode). Boot loader or similar code must ensure that the processor is in 24 bit contiguous addressing mode before calling the SDCC startup code. Like the --model-large option, variables will by default be placed into the XDATA segment. However, a current limitation is that the compiler will spill variables into the DATA segment when it runs out of registers. This means that compiling complex code can rapidly fill up the DATA segment. This limitation is being worked on, and should be addressed in the next release of sdcc. Segments may be placed anywhere in the 4 meg address space using the usual --*-loc options. Note that if any segments are located above 64K, the -r flag must be passed to the linker to generate the proper segment relocations, and the Intel HEX output format must be used. The -r flag can be passed to the linker by using the option -Wl-r on the sdcc command line. --stack-10bit: This option generates code for the 10 bit stack mode of the Dallas DS80C390 part. In this mode, the stack is located in the lower 1K of the internal RAM, which is mapped to 0x400000. With this option, sdcc will generate the proper addressing for stack variables. Note that the support is incomplete, since it still uses a single byte as the stack pointer. This means that only the lower 256 bytes of the potential 1K stack space can actually be used. However, this does allow you to reclaim the precious 256 bytes of low RAM for use for the DATA and IDATA segments. The compiler will not generate any code to put the processor into 10 bit stack mode. It is important to ensure that the processor is in this mode before calling any re-entrant functions compiled with this option. In principle, this should work with the --stack-auto option, but that has not been tested. It is incompatible with the --xstack option. It also only makes sense if the processor is in 24 bit contiguous addressing mode (see the --model-flat24 option).