This rom tests the bus-locking behavior of the SWP opcode.
A SWP opcode in ARMv4T is composed of a read cycle and a write cycle, in that order. Since SWP is typically used by software to implement thread-related mechanisms like semaphores, the CPU locks the bus throughout the read and write cycle, preventing anything else (e.g. DMA) from accessing the bus while the SWP is being executed.
This rom has a function in IWRAM that will run a DMA then immediately perform a SWP. The DMA and the SWP both write to the same word in memory. Because this function is in IWRAM, and because DMA takes 2 cycles to start up, the CPU has just enough time to begin executing the SWP opcode before the DMA fires. The first thing the SWP opcode does is its read cycle, so this read cycle is guaranteed to occur before the DMA. Then, one of two things can happen:
A) The SWP opcode locks the bus during execution, causing the DMA to occur after the SWP write and therefore overwrite whatever value the SWP wrote.
B) The SWP opcode does not lock the bus during execution, causing the SWP write to occur after the DMA and therefore overwrite whatever value the DMA wrote.
The DMA writes the value 0xCAFEBABE, and the SWP writes the value 0xDEADBEEF. The final written value will be displayed on screen when the test is run (it should display 0xCAFEBABE). Green background means you pass, red background means you fail.