diff --git a/fw/rtl/mcu/mcu_top.sv b/fw/rtl/mcu/mcu_top.sv
index 11ef5cc..a0dcc09 100644
--- a/fw/rtl/mcu/mcu_top.sv
+++ b/fw/rtl/mcu/mcu_top.sv
@@ -355,7 +355,8 @@ module mcu_top (
         REG_DD_SECTOR_INFO,
         REG_DD_DRIVE_ID,
         REG_VENDOR_SCR,
-        REG_VENDOR_DATA
+        REG_VENDOR_DATA,
+        REG_DEBUG
     } reg_address_e;
 
     logic bootloader_skip;
@@ -627,6 +628,13 @@ module mcu_top (
                 REG_VENDOR_DATA: begin
                     reg_rdata <= vendor_scb.data_rdata;
                 end
+
+                REG_DEBUG: begin
+                    reg_rdata <= {
+                        28'd0,
+                        n64_scb.pi_debug
+                    };
+                end
             endcase
         end
     end
diff --git a/fw/rtl/n64/n64_pi.sv b/fw/rtl/n64/n64_pi.sv
index b9f83fe..ade0326 100644
--- a/fw/rtl/n64/n64_pi.sv
+++ b/fw/rtl/n64/n64_pi.sv
@@ -250,7 +250,6 @@ module n64_pi (
     logic [15:0] read_fifo_rdata;
 
     logic read_fifo_wait;
-    logic [15:0] read_fifo_buffer;
 
     n64_pi_fifo read_fifo_inst (
         .clk(clk),
@@ -269,7 +268,10 @@ module n64_pi (
 
     always_ff @(posedge clk) begin
         read_fifo_read <= 1'b0;
-        read_fifo_buffer <= read_fifo_rdata;
+
+        if (reset) begin
+            n64_scb.pi_debug[1:0] <= 2'b00;
+        end
 
         if (reset || !pi_reset || alel_op) begin
             read_fifo_wait <= 1'b0;
@@ -279,16 +281,20 @@ module n64_pi (
             if (read_op) begin
                 if (read_fifo_empty) begin
                     read_fifo_wait <= 1'b1;
+                    n64_scb.pi_debug[0] <= 1'b1;
+                    if (read_fifo_wait) begin
+                        n64_scb.pi_debug[1] <= 1'b1;
+                    end
                 end else begin
                     read_fifo_read <= 1'b1;
-                    n64_pi_dq_out <= read_fifo_buffer;
+                    n64_pi_dq_out <= read_fifo_rdata;
                 end
             end
 
             if (!read_fifo_empty && read_fifo_wait) begin
                 read_fifo_read <= 1'b1;
                 read_fifo_wait <= 1'b0;
-                n64_pi_dq_out <= read_fifo_buffer;
+                n64_pi_dq_out <= read_fifo_rdata;
             end
         end
 
@@ -330,6 +336,10 @@ module n64_pi (
     always_ff @(posedge clk) begin
         write_fifo_write <= 1'b0;
 
+        if (reset) begin
+            n64_scb.pi_debug[3:2] <= 2'b00;
+        end
+
         if (reset) begin
             write_fifo_wait <= 1'b0;
         end
@@ -338,6 +348,10 @@ module n64_pi (
             if (write_op) begin
                 if (write_fifo_full) begin
                     write_fifo_wait <= 1'b1;
+                    n64_scb.pi_debug[2] <= 1'b1;
+                    if (write_fifo_wait) begin
+                        n64_scb.pi_debug[3] <= 1'b1;
+                    end
                 end else begin
                     write_fifo_write <= 1'b1;
                     write_fifo_wdata <= n64_pi_dq_in;
diff --git a/fw/rtl/n64/n64_scb.sv b/fw/rtl/n64/n64_scb.sv
index dae474f..3bf98fa 100644
--- a/fw/rtl/n64/n64_scb.sv
+++ b/fw/rtl/n64/n64_scb.sv
@@ -51,6 +51,8 @@ interface n64_scb ();
     logic [31:0] cfg_wdata [0:1];
     logic [31:0] cfg_version;
 
+    logic [3:0] pi_debug;
+
     modport controller (
         input n64_reset,
         input n64_nmi,
@@ -86,7 +88,9 @@ interface n64_scb ();
         input cfg_cmd,
         input cfg_rdata,
         output cfg_wdata,
-        output cfg_version
+        output cfg_version,
+
+        input pi_debug
     );
 
     modport pi (
@@ -105,7 +109,9 @@ interface n64_scb ();
 
         input flashram_read_mode,
 
-        input cfg_unlock
+        input cfg_unlock,
+
+        output pi_debug
     );
 
     modport flashram (
diff --git a/sw/controller/src/cfg.c b/sw/controller/src/cfg.c
index 5c639b4..559a1b6 100644
--- a/sw/controller/src/cfg.c
+++ b/sw/controller/src/cfg.c
@@ -519,6 +519,10 @@ void cfg_process (void) {
                 dd_set_sd_disk_info(args[0], args[1]);
                 break;
 
+            case '?':
+                args[0] = fpga_reg_get(REG_DEBUG);
+                break;
+
             default:
                 cfg_set_error(CFG_ERROR_UNKNOWN_CMD);
                 return;
diff --git a/sw/controller/src/fpga.h b/sw/controller/src/fpga.h
index c656f6a..63bdd9c 100644
--- a/sw/controller/src/fpga.h
+++ b/sw/controller/src/fpga.h
@@ -51,7 +51,8 @@ typedef enum {
     REG_DD_SECTOR_INFO,
     REG_DD_DRIVE_ID,
     REG_VENDOR_SCR,
-    REG_VENDOR_DATA
+    REG_VENDOR_DATA,
+    REG_DEBUG
 } fpga_reg_t;
 
 
diff --git a/sw/controller/src/usb.c b/sw/controller/src/usb.c
index a9ada86..386505f 100644
--- a/sw/controller/src/usb.c
+++ b/sw/controller/src/usb.c
@@ -275,6 +275,13 @@ static void usb_rx_process (void) {
                 }
                 break;
 
+            case '?':
+                p.rx_state = RX_STATE_IDLE;
+                p.response_pending = true;
+                p.response_info.data_length = 4;
+                p.response_info.data[0] = fpga_reg_get(REG_DEBUG);
+                break;
+
             default:
                 p.rx_state = RX_STATE_IDLE;
                 p.response_pending = true;
diff --git a/sw/pc/sc64.py b/sw/pc/sc64.py
index e5872f9..7fc56ca 100755
--- a/sw/pc/sc64.py
+++ b/sw/pc/sc64.py
@@ -193,6 +193,7 @@ class SC64Serial:
 
 class SC64:
     class __Address(IntEnum):
+        MEMORY = 0x0000_0000
         SDRAM = 0x0000_0000
         FLASH = 0x0400_0000
         BUFFER = 0x0500_0000
@@ -205,6 +206,7 @@ class SC64:
         SHADOW = 0x04FE_0000
 
     class __Length(IntEnum):
+        MEMORY = 0x0500_2980
         SDRAM = (64 * 1024 * 1024)
         FLASH = (16 * 1024 * 1024)
         BUFFER = (8 * 1024)
@@ -390,7 +392,7 @@ class SC64:
         self.__link.execute_cmd(cmd=b'U', args=[datatype, len(data)], data=data, response=False)
 
     def download_memory(self) -> bytes:
-        return self.__read_memory(self.__Address.SDRAM, self.__Length.SDRAM + self.__Length.FLASH)
+        return self.__read_memory(self.__Address.MEMORY, self.__Length.MEMORY)
 
     def upload_rom(self, data: bytes, use_shadow: bool=True) -> None:
         rom_length = len(data)
@@ -711,8 +713,9 @@ class EnumAction(argparse.Action):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='SC64 control software')
-    parser.add_argument('--backup-firmware', help='backup SC64 firmware and write it to specified file')
-    parser.add_argument('--update-firmware', help='update SC64 firmware from specified file')
+    parser.add_argument('--backup-firmware', metavar='file', help='backup SC64 firmware and write it to specified file')
+    parser.add_argument('--update-firmware', metavar='file', help='update SC64 firmware from specified file')
+    parser.add_argument('--bootloader', metavar='file', help='update SC64 bootloader (not recommended, use --update-firmware instead)')
     parser.add_argument('--reset-state', action='store_true', help='reset SC64 internal state')
     parser.add_argument('--print-state', action='store_true', help='print SC64 internal state')
     parser.add_argument('--boot', type=SC64.BootMode, action=EnumAction, help='set boot mode')
@@ -720,16 +723,15 @@ if __name__ == '__main__':
     parser.add_argument('--cic', type=SC64.CICSeed, action=EnumAction, help='force CIC seed to set value')
     parser.add_argument('--rtc', action='store_true', help='update clock in SC64 to system time')
     parser.add_argument('--no-shadow', action='store_false', help='do not put last 128 kB of ROM inside flash memory (can corrupt non EEPROM saves)')
-    parser.add_argument('--rom', help='upload ROM from specified file')
+    parser.add_argument('--rom', metavar='file', help='upload ROM from specified file')
     parser.add_argument('--save-type', type=SC64.SaveType, action=EnumAction, help='set save type')
-    parser.add_argument('--save', help='upload save from specified file')
-    parser.add_argument('--backup-save', help='download save and write it to specified file')
-    parser.add_argument('--ddipl', help='upload 64DD IPL from specified file')
-    parser.add_argument('--disk', action='append', help='path to 64DD disk (.ndd format), can be specified multiple times')
+    parser.add_argument('--save', metavar='file', help='upload save from specified file')
+    parser.add_argument('--backup-save', metavar='file', help='download save and write it to specified file')
+    parser.add_argument('--ddipl', metavar='file', help='upload 64DD IPL from specified file')
+    parser.add_argument('--disk', metavar='file', action='append', help='path to 64DD disk (.ndd format), can be specified multiple times')
     parser.add_argument('--isv', action='store_true', help='enable IS-Viewer64 support')
     parser.add_argument('--debug', action='store_true', help='run debug loop (required for 64DD and IS-Viewer64)')
-    parser.add_argument('--download-memory', help='download whole memory space and write it to specified file')
-    parser.add_argument('--bootloader', help='bootloader')
+    parser.add_argument('--download-memory', metavar='file', help='download whole memory space and write it to specified file')
 
     if (len(sys.argv) <= 1):
         parser.print_help()