All bank-switching information is stored on the caller's stack. This guarantees that a returning function call can switch back any banks that were switched to accommodate the call. This leaves the banks in their original state before the call was made.
A table is maintained to hold the current ROM pages for each of the switchable banks in RAM, along with the call nesting level. The call nesting level indicates the depth of API calls when an API call makes another API call before returning.
When any API call is made, irrespective of whether the call is made to an overlaid section of code, and bank switching is required, pertinent information is stored on the caller's stack. If the call is made to an overlaid area, the current ROM page in the table, along with other information is saved on the caller's stack, and the call nesting level is incremented. Only if the current ROM page does not match the ROM page required, is the ROM page actually banked in.
When the call returns, a check is made to see if the call was an overlay call. If the return was from an overlay call, the old ROM page on the stack is used to check if a bank-switch back is required. A bank-switch back is required if the current ROM page is not the same as the old RAM page on the stack, and if so, the bank is switched back in. The nesting level for the RAM page is decremented. To avoid unnecessary bank-switching back, a bank-switch back is performed only if the nesting level is greater than 1. This leaves the last bank-switch in, as the current bank.