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Debugging Spectrum Programs

Spectranext includes a powerful integrated debugger that allows you to debug ZX Spectrum programs using GDB or console commands. The debugger supports breakpoints, memory inspection, register access, and stack traces.

Overview​

The debugger provides:

  • GDB Stub: Connect GDB for full debugging support
  • Hardware Breakpoints: Break on specific memory addresses
  • Software Breakpoints: Break on code locations
  • Memory Inspection: Read/write memory
  • Register Access: View and modify CPU registers
  • Stack Traces: Inspect call stack
  • Source-level debugging: Debug C programs with z88dk-gdb
  • All with real hardware!

Compiling with Debug Symbols​

To enable source-level (C) debugging, compile your program with the -debug flag:

zcc +zx -debug main.c -create-app -m

This generates debugging symbols that allow GDB to map addresses to source code locations, enabling features like:

  • Setting breakpoints by function name or line number
  • Viewing variable values
  • Stepping through source code line by line
  • Viewing source code context

The -debug flag includes debugging information in the output files, which can be used with z88dk-gdb or standard GDB.

Assembly-Level Debugging​

You can debug any program at the assembly level, even without debug symbols. This is useful for:

  • Assembly programs: Programs written directly in Z80 assembly
  • Low-level debugging: Understanding what the CPU is actually executing
  • Optimization: Analyzing generated code from compilers
  • Reverse engineering: Understanding existing programs

Assembly Debugging Workflow​

  1. Load your program: Load the binary or TAP file (for example, see SDK)
  2. Set breakpoint: Set breakpoint at entry point or specific address 
    (gdb) b $8000
  3. Run to breakpoint: Start execution 
    (gdb) c
  4. Disassemble current code: See what's executing 
    (gdb) dis
  5. Step through instructions: Execute one instruction at a time 
    (gdb) si
  6. Inspect registers and memory: Check CPU state 
    (gdb) reg
  7. Continue or modify: Resume execution 
    (gdb) c

Useful Assembly Debugging Commands​

View memory in different formats:

(gdb) x $8000    # Hex dump

View stack:

(gdb) stack           # View stack contents

Example: Debugging Assembly Code​


# Set breakpoint at program start
(gdb) b $8000

# Run program
(gdb) c

# When breakpoint hits, disassemble
(gdb) dis
   0x8000: ld hl, 0x1234
   0x8003: ld de, 0x5678
   0x8006: add hl, de
   ...

# Step through instructions
(gdb) si
(gdb) reg

Assembly-level debugging gives you complete control over the CPU state and allows you to debug any program, regardless of whether it was compiled with debug symbols.

Debugger Architecture​

This section explains how the debugger itself functions in otder to serve its function. When the debugger traps (breakpoint hit or stop command), Spectranext remaps memory:

0x0000-0x0FFF   - Debugger 0-page code shadow page
0x1000-0x3FFF   - Unmapped
0x4000-0xFFFF   - Spectrum RAM

The debugger code keeps the Z80 running in a loop until a command is received from the debugger client. So while it might be perceived that Z80 is stopped, in fact it is running a small "halt" routine.

GDB Integration​

Spectranext's debugger can be accessed via GDB in several ways:

Connection Options​

For debugging on physical Spectranext hardware over USB-C:

The GDB debugger uses the single CDC interface named "spectranext". On macOS, the device appears as /dev/cu.usbmodem* (e.g., /dev/cu.usbmodem00015). On Linux, it's typically /dev/ttyACM0. The interface dynamically switches between console mode and GDB debugger mode based on the protocol detected.

Using z88dk-gdb:

z88dk-gdb -d /dev/cu.usbmodem00015 -x your_program.sym

Note: Spectranext exposes a single unified CDC interface that handles:

  • Console/Terminal access (default mode)
  • File transfer operations (via RSP protocol)
  • GDB debugging (automatically switches when GDB connects)

Using z88dk-gdb​

z88dk-gdb is a specialized GDB wrapper designed for Z80 debugging. Connect to the debugger:

# USB connection
z88dk-gdb -d <serial_device> -x <debug_symbols>

Parameters:

  • -d <device>: Serial device path for USB connections (e.g., /dev/cu.usbmodem00015 on macOS, /dev/ttyACM0 on Linux - the single CDC interface)
  • -h <host>: Hostname or IP address for network connections (e.g., 192.168.1.100). Useful when debugging with emulators, e.g. FuseX
  • -p <port>: Port number (only required for network connections)
  • -x <debug_symbols>: Path to debug symbols file generated during compilation

IDE Integration (CLion)​

You can debug Spectrum programs directly from CLion for a full-featured debugging experience with breakpoints, variable inspection, and step-through debugging.

Setup:

  1. Install z88dk: Ensure you have the latest z88dk installed (part of Spectranext SDK)
  2. Configure CMake: Use z88dk's CMake toolchain (see example below)
  3. Create Debug Configuration: Configure CLion to use z88dk-gdb as the debugger

CLion Debug Configuration:

  1. Go to Run > Edit Configurations...

  2. Create a new GDB Remote Debug configuration

  3. Set the following:

    • Name: Spectranext USB (or Spectranext WiFi)
    • GDB: Path to z88dk-gdb executable
    • Target: Your compiled binary
    • Symbol file: Your .map file (generated with -debug flag)
    • 'target remote' args:
      • For USB: /dev/cu.usbmodem00015 (or your device path)
      • For WiFi: <spectranext_ip>:1337
    • GDB startup commands: Leave empty or add custom initialization

  4. Set breakpoints in your source code

  5. Run the debug configuration - z88dk-gdb will connect and upload the binary

  6. The debugger will stop at your breakpoints

Example Project:

See the z88dk-gdb IDE test repository for a complete working example with CLion and VSCode configurations.

Emulators​

For debugging in emulators, use the emulator's GDB server:

MAME Emulator:

# Start MAME with GDB stub
mame spectrum -window -nomaximize -resolution0 768x576 -debug -debugger gdbstub -debugger_port 1337

# Connect z88dk-gdb
z88dk-gdb -h localhost -p 1337 -x your_program.sym

Fuse Emulator (Windows):

  1. Start Fuse
  2. Select "GDBServer..." from the menu
  3. Select "Enabled"
  4. Connect z88dk-gdb to the default port

Fuse Emulator (Mac):

  1. Start Fuse
  2. Go to "Preferences" > "Debugger"
  3. Check "Enable GDBServer"
  4. Connect z88dk-gdb to the default port

IDE Integration​

z88dk-gdb can be integrated with modern IDEs for a better debugging experience. See the IDE Integration (CLion) section above for detailed setup instructions.

Visual Studio Code:

  • Configure launch.json to use z88dk-gdb
  • Set breakpoints directly in the editor
  • View variables and call stack in the IDE
  • See the z88dk-gdb IDE test repository for VSCode configuration examples

CLion:

  • Configure remote debugger to use z88dk-gdb
  • Full debugging support with breakpoints and variable inspection
  • See the IDE Integration (CLion) section above for detailed setup instructions

z88dk-gdb Commands​

z88dk-gdb provides a rich set of debugging commands. Here are the most commonly used commands:

Execution Control​

  • continue / cont / c - Continue execution
  • step / s - Step one source line (including into calls)
  • stepi / si - Step one instruction (including into calls)
  • next / n - Step one source line (over calls)
  • nexti / ni - Step one instruction (over calls)
  • finish - Exit current function and print result if any

Breakpoints​

  • break / b [address/label] - Set breakpoint at address or label
  • delete / d [index] - Delete breakpoint

Memory and Register Inspection​

  • x [address] - Examine memory at address
  • print / p <expression> - Print expression value
  • registers / reg - Display all registers
  • info locals - Show local variables in current frame
  • info registers - Show register values

Stack and Frames​

  • backtrace / bt - Show execution stack
  • stack [size] - Examine stack (optionally specify size)
  • frame [num] - Set or view current frame
  • up - Go one frame up
  • down - Go one frame down

Disassembly​

  • disassemble / dis [address] [size] - Disassemble from address (or PC) for size bytes

Source Code​

  • list [address] - List source code around address or current location

Advanced Features​

  • restore <file> [address] - Restore memory from file
  • restore_pc <file> [address] - Restore memory and set PC – useful for uploading a complete program into ZX spectrum as well as setting the PC at the program's start
  • set <register/address> = <value> - Set register or memory value

Example z88dk-gdb Session​

# Connect to debugger over USB (recommended - single CDC interface)
z88dk-gdb -d /dev/cu.usbmodem00015 -x program.sym

# Or connect over WiFi/TCP
# z88dk-gdb -h <spectranext_ip> -p 1337 -x program.sym

# Set breakpoint
(gdb) break main

# Continue to breakpoint
(gdb) continue

# When stopped, inspect state
(gdb) info registers
(gdb) print variable_name
(gdb) x/16x 0x8000

# Step through code
(gdb) step
(gdb) next

# View stack
(gdb) backtrace
(gdb) stack 32

# Disassemble
(gdb) dis
(gdb) dis 0x8000 64

# Continue
(gdb) continue

IDE Integration​

You can debug Spectrum programs directly from modern IDEs like Visual Studio Code or CLion, providing a full-featured debugging experience with breakpoints, variable inspection, and step-through debugging.

See https://github.com/speccytools/z88dk-gdb-ide-test

Breakpoints​

Breakpoint Limitations

Breakpoint support varies by memory region:

  • RAM section (addresses ≥ 16KB): Up to 4 software breakpoints available
  • ROM section (addresses < 16KB): Only 1 hardware breakpoint available

Hardware Breakpoints​

Hardware breakpoints are implemented by Spectranext monitoring the Z80 address bus in real-time. Spectranext checks for a specific memory address on the bus during memory read cycles. The original instruction at the breakpoint address remains unchanged, so there's no code modification. Only 1 hardware breakpoint is available, but it has no performance impact and triggers immediately when the address matches. Hardware breakpoints work only in the ROM section (addresses < 16KB).

Software Breakpoints​

Software breakpoints modify the program code to insert a trap instruction. The instruction at the breakpoint address is replaced with RST $08 (0xCF), and the original instruction is saved and restored when the breakpoint is removed. Up to 4 software breakpoints are available, but they work only in the RAM section (addresses ≥ 16KB).

Example Debugging Session​

Complete Workflow​

  1. Compile with debug symbols:

    zcc +zx -debug main.c -create-app -m

  2. Start debugger target:

    • Physical hardware: Ensure Spectranext is running and accessible
    • Connect Spectranext to your computer over USB-C or have an active Wi-Fi

  3. Connect GDB:

    # USB connection (recommended - single CDC interface)
    z88dk-gdb -d /dev/cu.usbmodem00015 -x main.sym

  4. Set breakpoints:

    (gdb) b main
    (gdb) b main.c:20

  5. Start/continue execution:

    (gdb) continue
    (gdb) c

  6. Inspect state when breakpoint hits:

    (gdb) reg

  7. Step through code:

    (gdb) step      # Step into function calls
    (gdb) next      # Step over function calls
    (gdb) finish    # Finish current function

  8. Continue debugging or exit:

    (gdb) continue
    (gdb) quit

Limitations​

  • Limited breakpoints: Only 1 hardware, 4 software breakpoints
  • Debug symbols: Source-level debugging requires compiling with -debug flag, which disables certain program size optimizations.

Additional Resources​