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Z280 debugger

This is a simple debugger that illustrates the use of the hardware breakpoint-on-halt and single-step capabilities of the Zilog Z280 CPU.

The core of the debugger is based on the very simple debug monitor built into the RSX280 operating system. This version runs in standalone mode using the Z280 internal UART to communicate with the user's terminal, and assumes that the CPU is in Z80 mode (MMU disabled). That means also that the task to be debugged shares the same 64K address space with the debugger. Nevertheless, the code should not be difficult to modify so that it makes full use of the Z280 MMU and system/user modes. In fact, part of the code is already there (i.e. the MMU state is saved/restored when entering/leaving a trap, LDUP is used by the dump command to display memory from another bank, etc.)

The debugger includes a Z280 disassembler in a separate module that could be used in other applications.

As built, the debugger runs from address 0100h and uses memory up to 177Fh approximately (most of the space is taken up by the disassembler module); page zero is reserved for the Z280 interrupt/trap table. The code can be easily modified to run at a different address.

The debugger has been tested on Bill Shen's Z280RC board (a HEX file is included), and will be embedded into a future version of ZZmon.

Debugger commands

  • D addr1,addr2 -- Dump memory from addr1 to addr2 inclusive. If addr1 is omitted, the dump will continue from where the last dump operation stopped, or from the current contents of the user HL register after a breakpoint hit or a single-step operation. If addr2 is omitted, a full 256-byte page will be output.
  • L addr,n -- List (disassemble) n instructions starting from addr. If addr is not specified, the disassembly will continue from where the last L command ended, or from the current user PC after a breakpoint or single-step operation. If n is omitted, the next 16 instructions are displayed. Note that n is in hexadecimal.
  • E addr -- Examine/modify memory starting from addr. The address of the memory location is displayed, followed by the old contents. Entering a new value modifies the contents, pressing Enter on an empty line leaves the contents unchanged. End with Ctrl-C or with a dot.
  • F addr1,addr2,byte -- Fill memory region from addr1 to addr2 inclusive with byte. Any omitted parameter is assumed to be zero.
  • S addr1,addr2,byte -- Search memory region from addr1 to addr2 inclusive for byte. Any omitted parameter is assumed to be zero.
  • B addr -- Place a breakpoint (HALT instruction) at address addr. Up to eight breakpoints can be specified. Without the addr argument the command lists the current breakpoints.
  • C addr -- Clear breakpoint at addr.
  • G addr -- Go to address addr. Starts executing the user code until a breakpoint is hit. If addr is not specified, the execution will continue from the current user PC value.
  • T n -- Trace the next n instructions (one if n is not specified) starting from the current user PC value. Note that n is in hexadecimal.
  • I port -- Input value from port.
  • O port,value -- Output value to port.
  • W addr -- Display 16-bit word contents at the memory location addr.
  • X -- Display the contents of the CPU registers.
  • X rp -- Examine/modify CPU register pair rp.
  • XM -- Display the user MMU state.

Example usage on the Z280RC board

  1. Reset the Z280RC to get the ZZmon prompt, and send the debug180.hex file using your favorite terminal program.
  2. Load the program you want to debug, ensuring it will not overlay the debugger.
  3. Use the G0100 command to start the debugger.
  4. At the Z280> prompt, use the B command to set a breakpoint on your program.
  5. Start your program with the G command.
  6. When the breakpoint is hit, the CPU register contents will be displayed followed by the mnemonics of the instruction to be executed next (i.e. where the CPU stopped).
  7. Use the T command to single-step though the code, etc.

You can also start by setting the PC value with the XPC command followed by the T command to go directly into single-step mode, etc.