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I am looking into gdb, for breakpoint implementation. For ease I took the very first GDB release 2.51 (released 1988). I see the break point insert data as –
#define BREAKPOINT {0x4e, 0x4f}
what is 0x4e and 0x4f ?
The 0x4E 0x4F is the Motorola 68000 machine instruction for “TRAP #15”. The TRAP instruction forces an exception to occur, and on certain platforms trap #15 is defined as a breakpoint exception. This is why 0x4E and 0x4F appear in your GDB 2.51 source for handling platforms such as sun3.
References:
This Motorola M68000 Family Programmer’s Reference Manual contains the details of the TRAP instruction on page 4-188. Specifically, the instruction is represented by the 12-bit value 010011100100 followed by the 4-bit “vector” (in this case, 1111). So “TRAP #15” is represented by 0x4E 0x4F. Sun specifically uses vector 15 for breakpoint/tracing — a Google search reveals numerous comments and source code examples.