MAME Debugger¶

Introduction ¶

MAME includes an interactive low-level debugger that targets the emulated system. This can be a useful tool for diagnosing emulation issues, developing software to run on vintage systems, creating cheats, ROM hacking, or just investigating how software works.

Use the -debug command line option to start MAME with the debugger activated. By default, pressing the backtick/tilde (~) during emulation breaks into the debugger (this can be changed by reassigning the Break in Debugger input).

The exact appearance of the debugger depends on your operating system and the options MAME was built with. All variants of the debugger provide a multi-window interface for viewing the contents of memory and disassembled code.

The debugger console window is a special window that shows the contents of CPU registers and disassembled code around the current program counter address, and provides a command-line interface to most of the debugging functionality.

Debugger commands ¶

Debugger commands are described in the sections below. You can also type help <topic> in the debugger console, where <topic> is the name of a command, to see documentation directly in MAME.

Specifying devices and address spaces ¶

Many debugger commands accept parameters specifying which device to operate on. If a device is not specified explicitly, the CPU currently visible in the debugger is used. Devices can be specified by tag, or by CPU number:

Tags are the colon-separated paths that MAME uses to identify devices within a system. You see them in options for configuring slot devices, in debugger disassembly and memory viewer source lists, and various other places within MAME’s UI.
CPU numbers are monotonically incrementing numbers that the debugger assigns to CPU-like devices within a system, starting at zero. The cpunum symbol holds the CPU number for the currently visible CPU in the debugger (you can see it by entering the command print cpunum in the debugger console).

If a tag starts with a caret (^) or dot (.), it is interpreted relative to the CPU currently visible in the debugger, otherwise it is interpreted relative to the root machine device. If a device argument is ambiguously valid as both a tag and a CPU number, it will be interpreted as a tag.

Examples:

maincpu: The device with the absolute tag :maincpu.
^melodypsg: The sibling device of the visible CPU with the tag melodypsg.
.:adc: The child device of the visible CPU with the tag adc.
2: The third CPU-like device in the system (zero-based index).

Commands that operate on memory extend this by allowing the device tag or CPU number to be optionally followed by an address space identifier. Address space identifiers are tag-like strings. You can see them in debugger memory viewer source lists. If the address space identifier is omitted, a default address space will be used. Usually, this is the address space that appears first for the device. Many commands have variants with d, i and o (data, I/O and opcodes) suffixes that default to the address spaces at indices 1, 2 and 3, respectively, as these have special significance for CPU-like devices.

In ambiguous cases, the default address space of a child device will be used rather than a specific address space.

Examples:

ram: The default address space of the device with the absolute tag :ram, or the ram space of the visible CPU.
.:io: The default address space of the child device of the visible CPU with the tag io, or the io space of the visible CPU.
:program: The default address space of the device with the absolute tag :program, or the program space of the root machine device.
^vdp: The default address space of the sibling device of the visible CPU with the tag vdp.
^:data: The default address space of the sibling device of the visible CPU with the tag data, or the data space of the parent device of the visible CPU.
1:rom: The default address space of the child device of the second CPU in the system (zero-based index) with the tag rom, or the rom space of the second CPU in the system.
2: The default address space of the third CPU-like device in the system (zero-based index).

If a command takes an emulated memory address as a parameter, the address may optionally be followed by an address space specification, as described above.

Examples:

0220: Address 0220 in the default address space for the visible CPU.
0378:io: Address 0378 in the default address space of the device with the absolute tag :io, or the io space of the visible CPU.
1234:.:rom: Address 1234 in the default address space of the child device of the visible CPU with the tag :rom, or the rom space of the visible CPU.
1260:^vdp: Address 1260 in the default address space of the sibling device of the visible CPU with the tag vdp.
8008:^:data: Address 8008 in the default address space of the sibling device of the visible CPU with the tag data, or the data space of the parent device of the visible CPU.
9660::ram: Address 9660 in the default address space of the device with the absolute tag :ram, or the ram space of the root machine device.

The examples here include a lot of corner cases, but in general the debugger should take the most likely meaning for a device or address space specification.

Debugger expression syntax ¶

Expressions can be used anywhere a numeric or Boolean parameter is expected. The syntax for expressions is similar to a subset of C-style expression syntax, with full operator precedence and parentheses. There are a few operators missing (notably the ternary conditional operator), and a few new ones (memory accessors).

The table below lists all the operators, ordered from highest to lowest precedence:

( ): Standard parentheses
++ --: Postfix increment/decrement
++ -- ~ ! - + b@ w@ d@ q@ b! w! d! q!: Prefix increment/decrement, binary complement, logical complement, unary identity/negation, memory access
* / %: Multiplication, division, modulo
+ -: Addition, subtraction
<< >>: Bitwise left/right shift
< <= > >=: Less than, less than or equal, greater than, greater than or equal
== !=: Equal, not equal
&: Bitwise intersection (and)
^: Bitwise exclusive or
|: Bitwise union (or)
&&: Logical conjunction (and)
||: Logical disjunction (or)
= *= /= %= += -= <<= >>= &= |= ^=: Assignment and modifying assignment
,: Separate terms, function parameters

Major differences from C expression semantics:

All numbers are unsigned 64-bit values. In particular, this means negative numbers are not possible.
The logical conjunction and disjunction operators && and || do not have short-circuit properties – both sides of the expression are always evaluated.

Numbers ¶

Literal numbers are prefixed according to their bases:

Hexadecimal (base-16) with $ or 0x
Decimal (base-10) with #
Octal (base-8) with 0o
Binary (base-2) with 0b
Unprefixed numbers are hexadecimal (base-16).

Examples:

123 is 123 hexadecimal (291 decimal)
$123 is 123 hexadecimal (291 decimal)
0x123 is 123 hexadecimal (291 decimal)
#123 is 123 decimal
0o123 is 123 octal (83 decimal)
0b1001 is 1001 binary (9 decimal)
0b123 is invalid

Boolean values ¶

Any expression that evaluates to a number can be used where a Boolean value is required. Zero is treated as false, and all non-zero values are treated as true. Additionally, the string true is treated as true, and the string false is treated as false.

An empty string may be supplied as an argument for Boolean parameters to debugger commands to use the default value, even when subsequent parameters are specified.

Memory accesses ¶

The memory access prefix operators allow reading from and writing to emulated address spaces. The memory prefix operators specify the access size and whether side effects are disabled, and may optionally be preceded by an address space specification. The supported access sizes and side effect modes are as follows:

b specifies an 8-bit access (byte)
w specifies a 16-bit access (word)
d specifies a 32-bit access (double word)
q specifies a 64-bit access (quadruple word)
@ suppress side effects
! do not suppress side effects

Suppressing side effects of a read access yields the value reading from address would, with no further effects. For example reading a mailbox with side effects disabled will not clear the pending flag, and reading a FIFO with side effects disabled will not remove an item.

For write accesses, suppressing side effects doesn’t change behaviour in most cases – you want to see the effects of writing to a location. However, there are some exceptions where it is useful to separate multiple effects of a write access. For example:

Some registers need to be written in sequence to avoid race conditions. The debugger can issue multiple writes at the same point in emulated time, so these race conditions can be avoided trivially. For example writing to the MC68HC05 output compare register high byte (OCRH) inhibits compare until the output compare register low byte (OCRL) is written to prevent race conditions. Since the debugger can write to both locations at the same instant from the emulated machine’s point of view, the race condition is not usually relevant. It’s more error-prone if you can accidentally set hidden state when all you really want to do is change the value, so writing to OCRH with side effects suppressed does not inhibit compare, it just changes the value in the output compare register.
Writing to some registers has multiple effects that may be useful to separate for debugging purposes. Using the MC68HC05 as an example again, writing to OCRL changes the value in the output compare register, and also clears the output compare flag (OCF) and enables compare if it was previously inhibited by writing to OCRH. Writing to OCRL with side effects disabled just changes the value in the register without clearing OCF or enabling compare, since it’s useful for debugging purposes. Writing to OCRL with side effects enabled has the additional effects.

The size may optionally be preceded by an access type specification:

p or lp specifies a logical address defaulting to space 0 (program)
d or ld specifies a logical address defaulting to space 1 (data)
i or li specifies a logical address defaulting to space 2 (I/O)
3 or l3 specifies a logical address defaulting to space 3 (opcodes)
pp specifies a physical address defaulting to space 0 (program)
pd specifies a physical address defaulting to space 1 (data)
pi specifies a physical address defaulting to space 2 (I/O)
p3 specifies a physical address defaulting to space 3 (opcodes)
r specifies direct read/write pointer access defaulting to space 0 (program)
o specifies direct read/write pointer access defaulting to space 3 (opcodes)
m specifies a memory region

Finally, this may be preceded by a tag and/or address space name followed by a dot (.).

That may seem like a lot to digest, so let’s look at the simplest examples:

b@<addr>: Refers to the byte at <addr> in the program space of the current CPU while suppressing side effects
b!<addr>: Refers to the byte at <addr> in the program space of the current CPU, not suppressing side effects such as reading a mailbox clearing the pending flag, or reading a FIFO removing an item
w@<addr> and w!<addr>: Refer to the word at <addr> in the program space of the current CPU, suppressing or not suppressing side effects, respectively.
d@<addr> and d!<addr>: Refer to the double word at <addr> in the program space of the current CPU, suppressing or not suppressing side effects, respectively.
q@<addr> and q!<addr>: Refer to the quadruple word at <addr> in the program space of the current CPU, suppressing or not suppressing side effects, respectively.

Adding access types gives additional possibilities:

dw@300: Refers to the word at 300 in the data space of the current CPU while suppressing side effects
id@400: Refers to the double word at 400 in the I/O space of the current CPU while suppressing side effects
ppd!<addr>: Refers to the double word at physical address <addr> in the program space of the current CPU while not suppressing side effects
rw@<addr>: Refers to the word at address <addr> in the program space of the current CPU using direct read/write pointer access

If we want to access an address space of a device other than the current CPU, an address space beyond the first four indices, or a memory region, we need to include a tag or name:

ramport.b@<addr>: Refers to the byte at address <addr> in the ramport space of the current CPU
audiocpu.dw@<addr>: Refers to the word at address <addr> in the data space of the CPU with absolute tag :audiocpu
maincpu:status.b@<addr>: Refers to the byte at address <addr> in the status space of the CPU with the absolute tag :maincpu
monitor.mb@78: Refers to the byte at 78 in the memory region with the absolute tag :monitor
..md@202: Refers to the double word at address 202 in the memory region with the same tag path as the current CPU.

Some combinations are not useful. For example physical and logical addresses are equivalent for some CPUs, and direct read/write pointer accesses never have side effects. Accessing a memory region (m access type) requires a tag to be specified.

Memory accesses can be used as both lvalues and rvalues, so you can write b@100 = ff to store a byte in memory.

Functions ¶

The debugger supports a number of useful utility functions in expressions.

min(<a>, <b>): Returns the lesser of the two arguments.
max(<a>, <b>): Returns the greater of the two arguments.
if(<cond>, <trueval>, <falseval>): Returns <trueval> if <cond> is true (non-zero), or <falseval> otherwise. Note that the expressions for <trueval> and <falseval> are both evaluated irrespective of whether <cond> is true or false.
abs(<x>): Reinterprets the argument as a 64-bit signed integer and returns the absolute value.
bit(<x>, <n>[, <w>]): Extracts and right-aligns a bit field <w> bits wide from <x> with least significant bit position <n>, counting from the least significant bit. If <w> is omitted, a single bit is extracted.
s8(<x>): Sign-extends the argument from 8 bits to 64 bits (overwrites bits 8 through 63, inclusive, with the value of bit 7, counting from the least significant bit).
s16(<x>): Sign-extends the argument from 16 bits to 64 bits (overwrites bits 16 through 63, inclusive, with the value of bit 15, counting from the least significant bit).
s32(<x>): Sign-extends the argument from 32 bits to 64 bits (overwrites bits 32 through 63, inclusive, with the value of bit 31, counting from the least significant bit).