Implemented missing step instructions

Author: desertkun

src/crt0.asm

@@ -10,6 +10,7 @@ global __SYSVAR_BORDCR
 global _clear42
 global _print42
 global module_header
+global nmi_handler
 extern gdbserver_install
 
 defc INITIAL_SP = 0xFFFF

src/install.asm

@@ -68,7 +68,7 @@ rst8h_handler_src:
 rst8h_handler_src_end:
 
 STR_installed:
-    defb "*** GDBSERVER INSTALLED ***\nPress NMI, then attach on port 1667.\n", 0
+    defb "*** GDBSERVER v0.2 INSTALLED ***\nPress NMI, then attach on port 1667.\n", 0
 
 STR_install_error:
     defb "Cannot install. Make sure your spectranet firmware is updated.\n", 0
@@ -116,12 +116,29 @@ gdbserver_install_ok:
 
     jp EXIT_SUCCESS
 
-global _restore_rst08h
-_restore_rst08h:
+global _gdbserver_trap
+_gdbserver_trap:
+    # one time only
+    call DISABLETRAP
+
+    # restore rst8 on a breakpoint
+    ld a, (gdbserver_trap_flags)
+    and 0x01
+    jr z, gdbserver_trap_skip_restore_rst08
+
     # load instruction address
     ld hl, (gdbserver_trap_handler+5)
     # put rst 08 back
     ld (hl), 0xCF
-    # one time only
-    call DISABLETRAP
+gdbserver_trap_skip_restore_rst08:
+
+    # do a single instruction
+    ld a, (gdbserver_trap_flags)
+    and 0x02
+    jr z, gdbserver_trap_skip_trigger_step
+    # launch the debugger
+    extern nmi_handler
+    call nmi_handler
+gdbserver_trap_skip_trigger_step:
+
     jp PAGETRAPRETURN

src/modulecall.c

@@ -4,7 +4,7 @@
 #include "state.h"
 #include "sockpoll.h"
 
-extern void restore_rst08h();
+extern void gdbserver_trap();
 
 void modulecall()
 {
@@ -19,26 +19,38 @@ void modulecall()
         return;
     }
 
+    // reset breaking
+    gdbserver_state.trap_flags = 0;
+
     struct breakpoint_t* trapped_breakpoint = NULL;
+    uint16_t* pc = &gdbserver_state.registers[REGISTERS_PC];
 
     if (gdbserver_state.client_socket)
     {
         print42("execution stopped\n");
-        uint16_t* pc = &gdbserver_state.registers[REGISTERS_PC];
-
-        // unwind RST08
-        (*pc)--;
 
-        for (uint8_t i = 0; i < MAX_BREAKPOINTS_COUNT; i++)
+        if (gdbserver_state.temporary_breakpoint.address == (*pc - 1))
         {
-            struct breakpoint_t* b = &gdbserver_state.breakpoints[i];
-            if (b->address == *pc)
+            // we've hit temp breakpoint, original instruction
+            // note, we do not put RST08 back, because it is temporary
+            *(uint8_t*)gdbserver_state.temporary_breakpoint.address =
+                gdbserver_state.temporary_breakpoint.original_instruction;
+            // unwind RST08
+            (*pc)--;
+        }
+        else
+        {
+            for (uint8_t i = 0; i < MAX_BREAKPOINTS_COUNT; i++)
             {
-                trapped_breakpoint = b;
+                struct breakpoint_t* b = &gdbserver_state.breakpoints[i];
+                if (b->address == (*pc - 1)) // offset for RST08
+                {
+                    trapped_breakpoint = b;
 
-                // restore original instruction
-                *(uint8_t*)b->address = b->original_instruction;
-                break;
+                    // restore original instruction
+                    *(uint8_t*)b->address = b->original_instruction;
+                    break;
+                }
             }
         }
 
@@ -53,6 +65,12 @@ void modulecall()
         }
     }
 
+    if (trapped_breakpoint)
+    {
+        // unwind RST08
+        (*pc)--;
+    }
+
     while (1)
     {
         switch (poll_fd(gdbserver_state.client_socket))
@@ -83,13 +101,18 @@ void modulecall()
 done:
     print42("resuming execution\n");
 
-    // we have to restore RST08 on the breakpoint
     if (trapped_breakpoint && trapped_breakpoint->address == gdbserver_state.registers[REGISTERS_PC])
+    {
+        // we have to restore RST08 on the breakpoint
+        gdbserver_state.trap_flags |= TRAP_FLAG_RESTORE_RST08H;
+    }
+
+    if (gdbserver_state.trap_flags)
     {
         gdbserver_state.trap_handler.page = 0xFF;
-        gdbserver_state.trap_handler.address = trapped_breakpoint->address;
+        gdbserver_state.trap_handler.address = *pc;
         gdbserver_state.trap_handler.next_address = 0x0000;
-        gdbserver_state.trap_handler.handler = (uint16_t)restore_rst08h;
+        gdbserver_state.trap_handler.handler = (uint16_t)gdbserver_trap;
 
         set_trap(&gdbserver_state.trap_handler);
     }

src/server.c

@@ -122,6 +122,32 @@ static uint8_t process_packet()
             // continue execution
             return 0;
         }
+        case 's':
+        {
+            // do one step
+            gdbserver_state.trap_flags |= TRAP_FLAG_STEP_INSTRUCTION;
+            return 0;
+        }
+        case 'i':
+        {
+            // step over calls
+            uint8_t offset = *payload - '0'; // simple atoi
+
+            uint16_t address = gdbserver_state.registers[REGISTERS_PC] + offset;
+            gdbserver_state.temporary_breakpoint.address = address;
+            gdbserver_state.temporary_breakpoint.original_instruction = *(uint8_t*)address;
+
+            *(uint8_t*)address = 0xCF; // RST 08h
+
+            if (*(uint8_t*)address != 0xCF)
+            {
+                // write didn't do anything, probably read only
+                gdbserver_state.temporary_breakpoint.address = 0;
+                // so trip as soon as we can
+                gdbserver_state.trap_flags |= TRAP_FLAG_STEP_INSTRUCTION;
+            }
+            return 0;
+        }
         case 'q':
         {
             if (strstr(payload, "Xfer:features:read") == payload)

src/state.h

@@ -20,24 +20,37 @@ struct breakpoint_t {
 
 /*
  * Beware, only 1021 bytes are available
+ * Offsets of these are crucial
  */
 struct gdbserver_state_t
 {
+    // Offset: 0
     uint16_t registers[REGISTERS_COUNT]; /* sp, pc, hl, de, bc, af */
+    // Offset: 12
     uint8_t rst8_handler[80];
+    // Offset: 92
     int server_socket;
+    // Offset: 94
     int client_socket;
+    // Offset: 96
     struct {
         uint8_t page;
         uint16_t handler;
         uint16_t next_address;
         uint16_t address;
     } trap_handler;
+    // Offset: 103
+    uint8_t trap_flags;
+    // Offsets of these is not important
     uint8_t buffer[128];
     uint8_t w_buffer[128];
     struct breakpoint_t breakpoints[8];
+    struct breakpoint_t temporary_breakpoint;
 };
 
+#define TRAP_FLAG_RESTORE_RST08H (0x01)
+#define TRAP_FLAG_STEP_INSTRUCTION (0x02)
+
 extern struct gdbserver_state_t gdbserver_state;
 
 #endif

src/vars.inc

@@ -12,3 +12,4 @@ defc gdbserver_register_af = gdbserver_registers + 10
 defc gdbserver_rst8_handler = gdbserver_state + 12
 defc gdbserver_sockets = gdbserver_rst8_handler + 80
 defc gdbserver_trap_handler = gdbserver_sockets + 4
+defc gdbserver_trap_flags = gdbserver_trap_handler + 7