Editing Test-and-set (section)

=== Variation 2 ===
CPU 1 issues a test-and-set instruction to write to "memory location A".  The DPRAM does not immediately store the value in memory location A, but instead simultaneously moves the current value to a special register, while setting the contents of memory location A to a special "flag value".  If at this point, CPU 2 issues a test-and-set to memory location A, the DPRAM detects the special flag value, and as in Variation 1, issues a BUSY interrupt.

Whether or not CPU 2 was trying to access the memory location, the DPRAM now performs CPU 1's test.  If the test succeeds, the DPRAM sets memory location A to the value specified by CPU 1.  If the test fails, the DPRAM copies the value back from the special register to memory location A.  Either operation wipes out the special flag value.  If CPU 2 now issues a test-and-set, it will succeed.