program becomes unstable when exceeding 32kb

Yesterday I learned that my program can be 64kb big instead of just 32kb

Today I am having the problem that my program does not run when the program exceeds a certain size.

The machine I am programming does not finish it's boot cycle. And this is because timer 0 won't run. The machine at this point is stuck in a while loop, seeking connection with a HMI. Timer 0 ensures that this handshakingsprocedure runs. I've been using this handshaking for a long time on several machines without any issues.

So apparently when the program exceeds this 'limit' the timer ISR appears to be overwritten or something.

IDE-Version:
µVision V5.24.2.86
Copyright (C) 2017 ARM Ltd and ARM Germany GmbH. All rights reserved.

License Information:
sebastiaan knippels
Holland Mechanics b.v.

Tool Version Numbers:
Toolchain: C51 Compiler/Assembler Kit Version: 9.57.0.0
Toolchain Path: C:\Keil_v5\C51\BIN
C Compiler: C51.exe V9.57.0.0
Assembler: A51.exe V8.2.7.0
Linker/Locator: BL51.exe V6.22.2.0
Librarian: LIB51.exe V4.30.1.0
Hex Converter: OH51.exe V2.7.0.0
CPU DLL: S8051.DLL V3.113.0.0
Dialog DLL: DP51.DLL V2.64.0.0

The device I am programming is a virtual 8051 chip which is emulated on a FPGA chip. This hardware is fine and they have been programmed in assembly with programs exceeding 32kb.

Both my Off-chip code memory as well as my off-chip Xdata memory start at 0x0000 and have a size of 0xFFFF.

I would like to know why this happened and how I can fix this in Keil uVision. It is propably a certain target setting but I cannot seem to find it.

Momentarily I have set the compiler at level 9 code optimalization which produces a slightly smaller program. This produces fine code.... so far. I expect to get the same problem when my program grows again