The purpose of this page is to document the different commands of SameBoy’s textual debugger and its general usage and syntax

  1. General Usage
  2. Command Syntax
    1. Expressions
      1. Constants
      2. Variables
      3. Dereferencing
      4. Operators
  3. Commands

General Usage

SameBoy’s textual debugger is available in both the Cocoa and SDL ports, and allows advanced debugging of Game Boy ROMs. In the Cocoa port, the debugger can be access by enabling developer mode from the menu bar, and showing the console. In the SDL port, the debugger can be accessed by running SameBoy from the terminal, then it can be interacted with in the terminal. In the Windows SDL port, open the ROM with sameboy_debugger.exe instead of sameboy.exe.

In all versions, ^C (Control + C) breaks the execution of the ROM and allows input of debugger commands. Some commands can be used asynchronously without stopping the execution.

Command Syntax

Debugger commands begin with the command itself, followed optionally by a slash (/) and a modifier, and then followed optionally by a space and the argument.

For example, a command with no modifier and no argument:

continue

A command with a modifier and no argument:

disassemble/16

A command with an argument and no modifier:

print [de]

A command with both an argument and a modifier:

examine/64 hl

Commands may be truncated if not ambiguous (e.g. p instead of print) and might have aliases (x for examine)

Expressions

All commands that use a number or address as an argument process the argument using a an expression evaluator. Expressions can return 16-bit values or banked 25-bit addresses. The type of an expression affects the command that uses it. (For example, a breakpoint may be on a specific banked address, or every time pc reaches a specific value)

Constants

The most simple expression is a constant. Constants may be decimal numbers, hexadecimal numbers, or symbolic addresses. Decimal constants are simply written “as is” (e.g. 1024); hexadecimal constants are prefixed with a dollar sign (e.g. $400). Symbolic constants are simply written by name (e.g. PlayFXAnimID) and always return a banked address.

Variables

An expression can include any of these variables, which return their values:

  • Any 16-bit register: af, bc, de, hl, sp; these return 16-bit values
  • Any 8-bit register: a, f, b, c, d, e, h, l; these return their 8-bit values, zero-padded to 16-bits
  • The pc register, which return a 25-bit banked address
  • The special variables old and new which may be used in watchpoint conditions

Dereferencing

Any 16-bit or 25-bit may be dereferenced using either square brackets (e.g. [hl], [2:$4000]), to retrieve the 8-bit value at the specified address, zero-padded to 16-bit, or curly brackets (e.g. {hl}, {2:$4000}), to retrieve the the 16-bit value at the specified address.

Operators

Finally, expressions may be “joined” using operators. The return value depends on the operator, but is generally as follows:

  • Performing an operation on two 16-bit values return a 16-bit value
  • Performing an operation on a 16-bit value and a 25-bit address returns a 25-bit address
  • Performing an operation on two 25-bit address return a 16-bit value (Since adding, for example, pointers, has no meaning)
  • Boolean operations always return 16-bit bit values

The operators supported are:

  • +, -, *, /, % - The basic mathematical operators. Division or modulo by zero return zero.
  • |, &, ^ – Bitwise binary operators
  • ||, && – Boolean binary operators
  • <<, >> – Bitwise shift operators
  • <=, <, >, >=, ==, != – Boolean comparison operators
  • = – The assignment operator. The left side of the operator must be assignable: a register variable (e.g. pc, d) or a dereferenced value (e.g. [de + 2]). It returns the value of the left side after performing the write. When writing to hardware registers or the ROM, this might be different from the value written.
  • : – The bank operator. It can be used to cast or construct 25-bit addresses from 16-bit values. (e.g. $c:$437a, a:hl)

Operators are grouped into priority groups, like in classical mathematical order of operations. It is possible to use parentheses (( and )) to override the default order.

The priority groups are (First to last):

  • :
  • <=, <, >, >=, ==, !=
  • <<, >>
  • *, /, %, &, &&, ^
  • +, -, |, ||
  • =

Note that this order is different than the standard order of operations in C and other languages.

Commands

The information in this section is accessible inside SameBoy itself using the help command.

continue

Continue running until next stop

Usage: continue

next

Run the next instruction, skipping over function calls

Usage: next

step

Run the next instruction, stepping into function calls

Usage: step

finish

Run until the current function returns

Usage: finish

undo

Revert the last command

Usage: undo

registers

Print values of processor registers and other important registers

Usage: registers

backtrace

Display the current call stack

Usage: backtrace

Alias: bt

print

Evaluate and print an expression Use modifier to format as an address (a, default) or as a number in decimal (d), hexadecimal (x), octal (o) or binary (b).

Usage: print/format <expression>

Alias: eval

examine

Examine values at address

Usage: examine/count <expression>

Alias: x

disassemble

Disassemble instructions at address

Usage: disassemble/count <expression>

breakpoint

Add a new breakpoint at the specified address/expression Can also modify the condition of existing breakpoints. If the j modifier is used, the breakpoint will occur just before jumping to the target.

Usage: breakpoint/j <expression>[ if <condition expression>]

delete

Delete a breakpoint by its address, or all breakpoints

Usage: delete [<expression>]

watch

Add a new watchpoint at the specified address/expression. Can also modify the condition and type of existing watchpoints. Default watchpoint type is write-only.

Usage: watch/(r|w|rw) <expression>[ if <condition expression>]

unwatch

Delete a watchpoint by its address, or all watchpoints

Usage: unwatch [<expression>]

softbreak

Enable or disable software breakpoints (ld b, b opcodes)

Usage: softbreak (on|off)

list

List all set breakpoints and watchpoints

Usage: list

ticks

Display the number of CPU ticks since the last time 'ticks' was used

Usage: ticks

cartridge

Display information about the MBC and cartridge

Usage: cartridge

Alias: mbc

apu

Display information about the current state of the audio processing unit

Usage: apu [channel (1-4, 5 for NR5x)]

wave

Print a visual representation of the wave RAM. Modifiers can be used for a (f)ull print (the default), a more (c)ompact one, or a one-(l)iner

Usage: wave/(f|c|l)

lcd

Display information about the current state of the LCD controller

Usage: lcd

palettes

Display the current CGB palettes

Usage: palettes

dma

Display the current OAM DMA status

Usage: dma

help

List available commands or show help for the specified command

Usage: help [<command>]