I don't have an LC, but I looked at the ROM disassembly a bit. There are definitely limitations in ROM, and from the ROM, it is likely the VISA memory decoder is limited to 10mb.
The LC entry in the table the ROM has to tell it about the machine, says there are 2 banks of RAM: one up to 8mb, and one up to 2mb. The routine that figures out how much RAM is installed in each bank assumes always 2mb in the 2nd bank.
The LCII uses the same memory controller, but has 4mb soldered on. I also do not have an LCII, but everymac says it is limited to 10mb, although you can install up to 12mb (2x4mb SIMMs + 4mb soldered), but only 10mb will be recognized. Looking at the LCII ROM, the LC's table entry is unchanged, but the memory sizing routine now detects 4mb vs. 2mb soldered RAM. So, if you changed the LC's table entry to expand the 2nd bank size up to 4mb, it might be able to detect 4mb soldered. However, that doesn't expand the total memory available. In the case of an LCII with 12mb installed, the missing 2mb comes out of the soldered RAM, not the SIMMs. When sizing memory, the onboard RAM gets temporarily relocated to 8mb, meaning the max RAM that could possibly be detected, even if the various tables are updated, is 8mb. Additionally, when the 4mb of onboard LCII RAM gets relocated to 8mb, it can no longer access the top 2mb of that RAM, which seems to me to indicate a limitation in the VISA memory controller.
So, given all of that, it would appear the memory controller is limited to 10mb, but even if it is not, there are several places in the ROM that limit available RAM on the LC & LCII.
The LC entry in the table the ROM has to tell it about the machine, says there are 2 banks of RAM: one up to 8mb, and one up to 2mb. The routine that figures out how much RAM is installed in each bank assumes always 2mb in the 2nd bank.
The LCII uses the same memory controller, but has 4mb soldered on. I also do not have an LCII, but everymac says it is limited to 10mb, although you can install up to 12mb (2x4mb SIMMs + 4mb soldered), but only 10mb will be recognized. Looking at the LCII ROM, the LC's table entry is unchanged, but the memory sizing routine now detects 4mb vs. 2mb soldered RAM. So, if you changed the LC's table entry to expand the 2nd bank size up to 4mb, it might be able to detect 4mb soldered. However, that doesn't expand the total memory available. In the case of an LCII with 12mb installed, the missing 2mb comes out of the soldered RAM, not the SIMMs. When sizing memory, the onboard RAM gets temporarily relocated to 8mb, meaning the max RAM that could possibly be detected, even if the various tables are updated, is 8mb. Additionally, when the 4mb of onboard LCII RAM gets relocated to 8mb, it can no longer access the top 2mb of that RAM, which seems to me to indicate a limitation in the VISA memory controller.
So, given all of that, it would appear the memory controller is limited to 10mb, but even if it is not, there are several places in the ROM that limit available RAM on the LC & LCII.