Return zeros when GDB reads just past end of stack.

GDB likes to read the topmost value of stack when using the
"stepi" (step instruction) command. Unfortunately this does not work
when the stack is empty, causing an error message and GDB confusion.

Hack the GDB stub code to just return zeros if GDB tries to read two
bytes just past the end of stack.

NOTE: This may need to be modified to support also longer reads for
bigger AVRs where code pointers are longer than two bytes (e.g.
ATMega2560). As simavr doesn't support atm2560 yet, I'm implementing
it this way now.

diff --git a/simavr/sim/sim_gdb.c b/simavr/sim/sim_gdb.c
index f2d46ff..9bec4b5 100644 (file)
--- a/simavr/sim/sim_gdb.c
+++ b/simavr/sim/sim_gdb.c
@@ -208,6 +208,12 @@ static void gdb_handle_command(avr_gdb_t * g, char * cmd)
                                        gdb_send_reply(g, "E01");
                                        break;
                                }
+                       } else if (addr >= 0x800000 && (addr - 0x800000) == avr->ramend+1 && len == 2) {
+                               // Allow GDB to read a value just after end of stack.
+                               // This is necessary to make instruction stepping work when stack is empty
+                               printf("GDB read just past end of stack %08x, %08x; returning zero\n", addr, len);
+                               gdb_send_reply(g, "0000");
+                               break;
                        } else {
                                printf("read memory error %08x, %08x (ramend %04x)\n", addr, len, avr->ramend+1);
                                gdb_send_reply(g, "E01");