.\" $OpenBSD: ddb.4,v 1.77 2015/02/16 16:38:54 naddy Exp $ .\" $NetBSD: ddb.4,v 1.5 1994/11/30 16:22:09 jtc Exp $ .\" .\" Mach Operating System .\" Copyright (c) 1991,1990 Carnegie Mellon University .\" All Rights Reserved. .\" .\" Permission to use, copy, modify and distribute this software and its .\" documentation is hereby granted, provided that both the copyright .\" notice and this permission notice appear in all copies of the .\" software, derivative works or modified versions, and any portions .\" thereof, and that both notices appear in supporting documentation. .\" .\" CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" .\" CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR .\" ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE. .\" .\" Carnegie Mellon requests users of this software to return to .\" .\" Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU .\" School of Computer Science .\" Carnegie Mellon University .\" Pittsburgh PA 15213-3890 .\" .\" any improvements or extensions that they make and grant Carnegie Mellon .\" the rights to redistribute these changes. .\" .Dd $Mdocdate: February 16 2015 $ .Dt DDB 4 .Os .Sh NAME .Nm ddb .Nd kernel debugger .Sh DESCRIPTION The .Nm debugger provides a means for debugging the kernel, and analysing the kernel after a system crash ("panic"), with a .Xr gdb 1 Ns \&-like syntax. .Pp .Nm is invoked upon a kernel panic when the .Xr sysctl 8 .Va ddb.panic is set to 1. It may be invoked from the console when the sysctl .Va ddb.console is set to 1, using any of the following methods: .Bl -dash -offset 3n .It Using the key sequence .Li Ctrl-Alt-Esc . .It Sending a .Dv BREAK when using a serial console. .It Writing to the sysctl .Va ddb.trigger . .It For i386 and amd64 architectures, using the key sequence .Li Ctrl-Alt-Delete when the sysctl .Va machdep.kbdreset is set to 2. .El .Pp .Nm prompts for commands on the console with: .Pp .Dl ddb> .Pp The general syntax of a .Nm command is: .Bd -ragged -offset indent .Ar command .Oo Ic / Ns Ar modifiers Oc " " .Oo Ar address Oc Ns .Op Ic \&, Ns Ar count .Ed .Pp To save typing, .Nm makes use of a context inferred from previous commands. In this context, the current location is called .Va dot . .\" The .\" .Va dot .\" is displayed with .\" a hexadecimal format at a prompt. The .Ic examine , .Ic search , .Ic show struct , and .Ic write commands update .Va dot to be that of the last address examined or the last location modified, and have intuitive effects on .Va next and .Va prev . All the other commands do not change .Va dot , and set .Va next to be the same. (See .Sx VARIABLES . ) .Pp .\" Specifying .\" .Ar address .\" in a command sets .\" .Va dot . An expression can be used in place of .Ar address (see .Sx EXPRESSIONS ) . Omitting .Ar address in a command uses the last value of .Va dot . A missing .Ar count is taken to be 1 for printing commands or \*(If for stack traces. Entering a blank line causes the last command to be repeated using .Va next in place of .Ar address , a .Ar count of 1, and no modifiers. .Pp .Nm has a feature like .Xr more 1 for the output. If the number of lines output in response to one command exceeds the number set in the .Va $lines variable, it displays the message .Ql "--db_more--" and waits for a response. .Pp The valid responses are: .Pp .Bl -tag -width 10n -offset indent -compact .It Aq Ic space One more page. .It Aq Ic return One more line. .It Ic q Abort the current command, and return to the command input mode. .El .Pp The following command line editing keys are provided: .Pp .Bl -tag -width 10n -offset indent -compact .It Ic ^b back one character .It Ic ^f forward one character .It Ic ^a beginning of line .It Ic ^e end of line .It Ic ^w erase word back .It Ic ^h | Aq Ic del erase previous character .It Ic ^d erase next character .It Ic ^k delete to end of line .It Ic ^u delete line .It Ic ^p previous in command history .It Ic ^n next in command history .It Ic ^r redraw line .It Ic ^t exchange the two characters to the left of the cursor .El .Sh COMMANDS The following commands may be typed at the .Ql ddb> prompt. Some commands consist of more than one word, and if only the first word or words are entered, the possible alternatives to complete the command are displayed and no other action is performed. .Bl -tag -width 10n .\" -------------------- .It Ic help List the available commands. .\" -------------------- .It Xo .Oo Ic e Oc Ns .Ic x Ns Op Ic amine .Op Cm /bhlqaAxzodurcsmiI .Op Ar addr Ns .Op Ic \&, Ns Ar count .Xc Display the contents at address .Ar addr according to the formats in the modifier. If no format is specified, the last formats specified for this command are used. .Pp The format characters are: .Pp .Bl -tag -width 4n -compact .It Cm /b look at by bytes (8 bits) .It Cm /h look at by half words (16 bits) .It Cm /l look at by long words (32 bits) (default) .It Cm /q look at by long longs (64 bits) (only available on 64-bit platforms) .It Cm /a print the location being displayed .It Cm /A print the location with a line number if possible .It Cm /x display in unsigned hex .It Cm /z display in signed hex .It Cm /o display in unsigned octal .It Cm /d display in signed decimal .It Cm /u display in unsigned decimal .It Cm /r display in current radix, signed .It Cm /c display low 8 bits as a character. Non-printing characters are displayed as an octal escape code (e.g., '\e000'). .It Cm /s display the null-terminated string at the location. Non-printing characters are displayed as octal escapes. .It Cm /m display in unsigned hex with character dump at the end of each line. The location is also displayed in hex at the beginning of each line. .It Cm /i display as an instruction .It Cm /I display as an alternate format instruction depending on the machine: .Pp .Bl -tag -width powerpc_ -compact .It alpha Print affected register contents for every instruction. .It amd64 , .It i386 Do not skip padding to the next long word boundary for unconditional jumps. .It m88k Decode instructions for the opposite CPU model (e.g. m88110 when running on an m88100 processor). .It vax Don't assume that each external label is a procedure entry mask. .El .El .Pp The value of .Va next is set to the .Ar addr plus the size of the data examined. .\" -------------------- .It Xo .Ic print .Op Cm /axzodurc .Op Ar addr Op Ar addr ... .Xc Print each .Ar addr according to the modifier character. The valid modifiers are a subset of those from the .Ic examine command, and act as described there. If no modifier is specified, the last one specified in a previous use of .Ic print is used. The .Ar addr argument can be a string, and it is printed as a literal. .Pp For example, .Bd -literal -offset indent print/x "eax = " $eax "\enecx = " $ecx "\en" .Ed .Pp will print something like this: .Bd -literal -offset indent eax = xxxxxx ecx = yyyyyy .Ed .\" -------------------- .\" .It Xo Ic w Ns Op Cm /bhl .\" .Op Ar addr .\" .Ar expr Op expr ... .\" .Xc .It Xo .Ic w Ns Op Ic rite .Op Cm /bhl .Op Ar addr .Ar expr Op Ar expr ... .Xc Write the value of each .Ar expr expression at succeeding locations start at .Ar addr . The write unit size can be specified using one of the modifiers: .Pp .Bl -tag -width 4n -offset indent -compact .It Cm /b byte (8 bits) .It Cm /h half word (16 bits) .It Cm /l long word (32 bits) (default) .El .Pp The value of .Va next is set to .Ar addr plus the size of values written. .Pp .Sy Warning: since there is no delimiter between expressions, the command may not parse as you expect. It is best to enclose each expression in parentheses. .\" -------------------- .It Xo Ic set .Ic $ Ns Ar name .Op Ic = .Ar expr .Xc Set the named variable or register with the value of .Ar expr . Valid variable names are described below. .\" -------------------- .It Ic boot Ar how Reboot the machine depending on .Ar how : .Pp .Bl -tag -width "boot poweroff" -compact .It Ic boot sync Sync disks and reboot. .It Ic boot crash Dump core and reboot. .It Ic boot dump Sync disks, dump core and reboot. .It Ic boot halt Just halt. .It Ic boot reboot Just reboot. .It Ic boot poweroff Power down the machine whenever possible; if it fails, just halt. .El .\" -------------------- .It Xo .Ic break .Op Ar addr Ns .Op Ic \&, Ns Ar count .Xc Set a break point at .Ar addr . If .Ar count is supplied, .Nm allows the breakpoint to be silently hit .Ar ( count No \- 1 ) times before stopping at the break point. .Pp If the break point is successfully set, a break point number is displayed, in the form .Ic # Ns Ar number . This can later be used in deleting the break point or for adding conditions to it. .\" -------------------- .\" .It Xo Ic d .\" .Op Ar addr | Ic # Ns Ar number .\" .Xc .It Xo .Ic d Ns Op Ic elete .Op Ar addr | Ic # Ns Ar number .Xc Delete the break point set with the .Ic break command. .\" -------------------- .\" .It Xo Ic s Ns Op Cm /p .\" .Op Ic \&, Ns Ar count .\" .Xc .It Xo .Ic s Ns Op Ic tep .Op Cm /p .Op Ic \&, Ns Ar count .Xc Single step .Ar count times. If the .Cm /p modifier is specified, print each instruction at each step. Otherwise, only print the last instruction. .Pp .Sy Warning: depending on machine type, it may not be possible to single-step through some low-level code paths. On machines with software-emulated single-stepping (e.g., alpha), stepping through code executed by interrupt handlers will probably do the wrong thing. .\" -------------------- .It Ic call Ar name Ns Xo .Ic \&( Ns Ar expr .Op Ic \&, Ar expr ... .Ic \&) .Xc Call the function named by .Ar name with the argument(s) listed in parentheses. Parentheses may be omitted if the function takes no arguments. The number of arguments is currently limited to 10. .\" -------------------- .\" .It Ic c Ns Op Cm /c .It Xo .Ic c Ns Op Ic ontinue .Op Cm /c .Xc Continue execution until a breakpoint or watchpoint. If the .Cm /c modifier is given, instructions are counted while executing. .Pp .Sy Warning: when counting with .Cm /c , .Nm is really silently single-stepping. This means that single-stepping on low-level code may cause strange behavior. .\" -------------------- .It Xo .Ic watch .Ar addr .Op Ic \&, Ns Ar size .Xc Set a watchpoint for the region starting at .Ar addr . Execution stops and control returns to .Nm when an attempt is made to modify a watched region. The .Ar size argument defaults to 4. .Pp If you specify a wrong space address, the request is rejected with an error message. .Pp .Sy Warning: attempts to watch wired kernel memory may cause an unrecoverable error on some systems (e.g., i386). .\" -------------------- .It Ic dwatch Ar addr Delete the watchpoint at address .Ar addr that was previously set with a .Ic watch command. .\" -------------------- .It Xo .Ic hangman .Op Cm /s Ns Op Ic 0-9 .Xc This is a tiny and handy tool for random kernel hangs analysis, of which its depth is controlled by the optional argument of the default value of five. It uses some sophisticated heuristics to spot the global symbol that caused the hang. Since the discovering algorithm is a probabilistic one, you may spend substantial time to figure the exact symbol name. This smart thing requires a little of your attention, the input it accepts is mostly of the same format as that of the famous .Xr hangman 6 game, to which it, apparently, is obliged by the name. Hint: the .Xr nm 1 utility might help. .\" -------------------- .It Ic until Op Cm /p Stop at the next .Qq call or .Qq return instruction. If the .Cm /p modifier is specified, .Nm prints the call nesting depth and the cumulative instruction count at each call or return. Otherwise, it stays silent until the matching return is hit. .\" -------------------- .It Ic match Op Cm /p Stop at the next matching return instruction. If the .Cm /p modifier is specified, .Nm prints the call nesting depth and the cumulative instruction count at each call or return. Otherwise, it remains mostly quiet. .\" -------------------- .It Ic next Op Cm /p The .Ic next command is a synonym for .Ic match . .\" -------------------- .It Xo .Ic trace .Op Cm /pu .Op Ar frameaddr Ns .Op Ic \&, Ns Ar count .Xc Show the stack trace. The .Cm /p modifier interprets the .Ar frameaddr argument as the PID of a process and shows the stack trace of that process. The .Cm /p modifier is not supported on all platforms. The .Cm /u modifier shows the stack trace of user space; if omitted, the kernel stack is traced instead. The .Ar count argument is the limit on the number of frames to be followed. If .Ar count is omitted, all frames are printed. .Pp .Sy Warning: user space stack trace is valid only if the machine dependent code supports it. .\" -------------------- .It Xo .Ic search .Op Cm /bhl .Op Ar addr .Ar value .Op Ar mask .Op Ic \&, Ns Ar count .Xc Search memory for a value beginning at .Ar addr . This command might fail in interesting ways if it doesn't find the searched-for value. This is because .Nm doesn't always recover from touching bad memory. The optional .Ar count argument limits the search. The modifiers are the same as those of the .Ic write command. .Pp The .Va next address is set to the address where .Ar value is found, or just after where the search area finishes. .\" -------------------- .It Ic show Ar what Displays various things, depending on .Ar what : .Bl -tag -width 4n .\" -------------------- .It Ic show bcstats Prints the buffer cache statistics. .\" -------------------- .It Ic show breaks Prints a list of all breakpoints that have been set with the .Ic break command. .\" -------------------- .It Xo .Ic show buf .Op Cm /f .Ar addr .Xc Prints the .Li struct buf at .Ar addr . If the .Cm /f modifier is specified output will also include .Li softdep printout, if those are available. .\" -------------------- .It Ic show extents Prints a detailed list of all extents. .\" -------------------- .It Ic show malloc Op Ar addr Prints malloc debugging information if available. If an optional address is specified, only information about that address is printed. .\" -------------------- .It Xo .Ic show map .Op Cm /f .Ar addr .Xc Prints the .Li vm_map at .Ar addr . If the .Cm /f modifier is specified the complete map is printed. .\" -------------------- .It Ic show mbuf Ar addr Prints the .Li struct mbuf header at .Ar addr . Depending on the mbuf flags .Li struct pkthdr and .Li struct m_ext are printed as well. .\" -------------------- .It Xo .Ic show mount .Op Cm /f .Ar addr .Xc Prints the .Li struct mount at .Ar addr . If the .Cm /f modifier is specified prints out all .Li vnodes (see also .Cm show vnode ) and also all .Li bufs (see also .Cm show buf ) on all those vnodes. .\" -------------------- .It Xo .Ic show nfsnode .Op Cm /f .Ar addr .Xc Prints the .Li struct nfsnode at .Ar addr . If the .Cm /f modifier is specified prints out additional information as well. .\" -------------------- .It Xo .Ic show nfsreq .Op Cm /f .Ar addr .Xc Prints the .Li struct nfsreq at .Ar addr . If the .Cm /f modifier is specified prints out additional information as well. .\" -------------------- .It Xo .Ic show object .Op Cm /f .Ar addr .Xc Prints the .Li vm_object at .Ar addr . If the .Cm /f modifier is specified the complete object is printed. .\" -------------------- .It Xo .Ic show offset .Ar offset .Xc Prints a list of the known kernel structure fields which occur at the given .Ar offset from the beginning of the struct, as well as their size. The .Cm option DDB_STRUCT is required for this command to be available. .\" -------------------- .It Xo .Ic show page .Op Cm /f .Ar addr .Xc Prints the .Li vm_page at .Ar addr . If the .Cm /f modifier is specified the complete page is printed. .\" -------------------- .It Ic show panic Prints the panic string. .\" -------------------- .It Xo .Ic show pool .Op Cm /p .Ar addr .Xc Prints the .Li pool at .Ar addr . Valid modifiers: .Bl -tag -width xxx -compact .It Cm /p Print the pagelist for this pool. .El .\" -------------------- .It Ic show proc Op Ar addr Prints the .Li struct proc at .Ar addr . If an optional address is not specified .Li curproc is assumed. .\" -------------------- .It Ic show registers Op Cm /u Display the register set. If the .Cm /u modifier is specified, it displays user registers (or the currently saved registers) instead of the kernel's. Note: The .Cm /u modifier is not supported on every machine, in which case incorrect information may be displayed. .\" -------------------- .It Xo .Ic show struct .Ar name .Op addr .Xc Prints the content of the memory at .Ar addr as a struct .Ar name . Nested structures and bit fields are not printed. Character arrays are printed as bytes. The .Cm option DDB_STRUCT is required for this command to be available. .\" -------------------- .It Ic show uvmexp Displays a selection of uvm counters and statistics. .\" -------------------- .It Xo .Ic show vnode .Op Cm /f .Ar addr .Xc Prints the .Li struct vnode at .Ar addr . If the .Cm /f modifier is specified prints out all .Li bufs (see also .Cm show buf ) currently attached to this .Li vnode . .\" -------------------- .It Ic show watches Displays all watchpoints set with the .Ic watch command. .\" -------------------- .It Ic show all procs Op Cm /anw Display information on all processes. .Pp .Bl -tag -width foo -compact .It Cm /n (Default) Show process information in a .Xr ps 1 Ns \&-like format. Information printed includes process ID, parent process ID, process group, UID, process status, process flags, process command name, and process wait channel message. .It Cm /a Shows the kernel virtual addresses of each process' proc structure, u-area, and vmspace structure. The vmspace address is also the address of the process' .Li vm_map structure and can be used in the .Ic show map command. .It Cm /w Shows each process' PID, command, system call emulation, wait channel address, and wait channel message. .El .\" -------------------- .It Ic show all bufs Op Cm /f Display information about all buffers in the system. .Pp .Bl -tag -width foo -compact .It Cm /f For each buffer, print a more detailed output. See the .Ic show buf command for more information. .El .\" -------------------- .It Ic show all callout Display the contents of the callout table. .\" -------------------- .It Ic show all pools Op Cm /a Display information about all system pools in a format similar to .Xr vmstat 8 . .Pp .Bl -tag -width foo -compact .It Cm /a Displays .Dq interesting address information. .El .\" -------------------- .It Ic show all mounts Op Cm /f Display information on all mounted filesystems. .Pp .Bl -tag -width foo -compact .It Cm /f For each filesystem, list all its struct vnode addresses. These addresses can be used in the .Ic show vnode command. .El .\" -------------------- .It Ic show all nfsnodes Op Cm /f Display information about all nfsnodes in the system. .Pp .Bl -tag -width foo -compact .It Cm /f For each nfsnode, print a more detailed output. See the .Ic show nfsnode command for more information. .El .\" -------------------- .It Ic show all nfsreqs Op Cm /f Display information for all outstanding NFS requests. .Pp .Bl -tag -width foo -compact .It Cm /f For each NFS requests, print a more detailed output. See the .Ic show nfsreq command for more information. .El .\" -------------------- .It Ic show all vnodes Op Cm /f Display information about all vnodes in the system. .Pp .Bl -tag -width foo -compact .It Cm /f For each vnode, print a more detailed output. See the .Ic show vnode command for more information. .El .El .\" -------------------- .It Ic callout A synonym for the .Ic show all callout command. .\" -------------------- .It Ic ps Op Cm /anw A synonym for .Ic show all procs . .\" -------------------- .It Xo .Ic mac Ns Op Ic hine .Ar subcommand Op Ar args ... .Xc Perform a platform-specific command. .Pp The following commands are supported by multiprocessor kernels on these platforms: amd64, i386, macppc, and mips64. .Pp .Bl -tag -width "searchframe [addr]" -compact .It Ic cpuinfo Display the state of each CPU. .It Ic ddbcpu Ar N Stop the current CPU and start handling .Nm on the selected CPU. .It Ic startcpu Op Ar N Resume normal processing on the selected CPU, or all CPUs if none is specified. .It Ic stopcpu Op Ar N Stop normal processing on the selected CPU, or all CPUs (except the one handling .Nm ) if none is specified. .El .Pp Other platform-specific commands: .\" .Pp .\" amd64: .Pp arm: .Bl -tag -width "searchframe [addr]" -compact .It Ic frame Ar addr Display the trapframe at .Ar addr . .El .Pp i386: .Bl -tag -width "searchframe [addr]" -compact .\" .It Ic acpi disasm Ar ?? .\" .It Ic acpi showval Ar ?? .\" .It Ic acpi tree Ar ?? .\" .It Ic acpi trace Ar ?? .\" Evil ACPI debugging magic. .It Ic sysregs Display the contents of the privileged registers: .Va IDTR , .Va GDTR , .Va LDTR , .Va TR , .Va CR0 , .Va CR2 , .Va CR3 , and .Va CR4 . .\" (i386-only) .El .Pp m88k: .Bl -tag -width "searchframe [addr]" -compact .It Ic ddbcpu Ar N Stop the current CPU and start handling .Nm on the selected CPU. .It Ic frame Ar addr Display the trapframe at .Ar addr . .It Ic regs Display the registers from when .Nm was entered. .It Ic searchframe Op Ar addr Search for and display stack exception frames, starting from .Ar addr if given, else the address in register .Va r31 , and stopping at the next 8k boundary. .It Ic where Display where the current CPU was stopped. .El .Pp mips64: .Bl -tag -width "searchframe [addr]" -compact .It Xo .Ic tlb Op Cm /p Ar asid .Op Cm /c .Op Ar tlb .Xc .It Ic trap Ar ?? .El .Pp sh: .Bl -tag -width "searchframe [addr]" -compact .It Ic cache Op Ar addr Display the cache, starting from .Ar addr , defaulting to 0. .It Ic frame Display the switch and trap frames. .\" .It Ic stack .It Ic tlb Display the TLB. .El .Pp sparc: .Bl -tag -width "searchframe [addr]" -compact .It Ic proc Ar ?? .El .Pp sparc64: .Bl -tag -width "searchframe [addr]" -compact .It Ic ctx Display the context addresses for all threads. .\" .It Ic dtlb Ar 0,2 .\" .It Ic dtsb .\" .It Ic esp .\" .It Ic fpstate Op Ar addr .\" .It Ic itlb Ar 0,1,2 .\" .It Ic kmap Op Ar addr .\" .It Ic pcb Op Ar addr .\" .It Ic pctx Ar pid .\" .It Ic page Ar addr .\" .It Ic phys Op Ar addr .\" .It Ic pmap Op Ar addr .\" .It Ic proc Op Ar addr .\" .It Ic prom Ar ?? .\" .It Ic pv Ar addr .\" .It Ic stack Op Ar addr .\" .It Ic tf Op Ar addr .\" .It Ic ts .\" .It Ic traptrace Op Ar addr .\" .It Ic watch Op Ar addr .\" .It Ic window Op Ar winnum .\" .It Ic xir Op Ar addr .El .\" -------------------- .El .Sh VARIABLES .Nm denotes registers and variables by .Ic $ Ns Va name . Register names can be found with the .Ic show registers command. .Pp Some variable names are suffixed with numbers, and some may have a modifier following a colon immediately after the variable name. For example, register variables can have the .Ql :u modifier to indicate a user register (e.g., .Ql $eax:u ) . .Pp Built-in debugger variables currently supported are: .Bl -tag -width 10n -compact -offset indent .It Va $radix Input and output radix. .It Va $maxoff Addresses are printed as .Ar symbol Ns Li + Ns Ar offset unless .Ar offset is greater than .Va $maxoff . .It Va $maxwidth The width of the displayed lines. .It Va $lines The number of lines to page. This is used by the .Dq more feature. .It Va $tabstops Tab stop width. .It Va $log Controls whether the output of .Nm will also appear in the system message buffer. .El .Pp These variables can also be controlled outside .Nm via the .Sq ddb .Xr sysctl 8 hierarchy. .Sh EXPRESSIONS Almost all expression operators in C are supported except for .Ql ~ , .Ql ^ , and unary .Ql & . Special rules for expressions in .Nm are: .Bl -tag -width 15n -compact -offset indent .It Ar identifier The name of a symbol. It is translated to the address (or value) of the symbol. .Ql \&. and .Ql \&: can be used in the identifier. The following can be accepted as an identifier, if supported by an object format dependent routine: .Bl -item -offset indent -compact .It .Sm off .Oo Ar filename Li \&: Oc Ar func .Op \&: Ar linenumber .It .Op Ar filename \&: .Ar variable .It .Ar filename .Op \&: Ar linenumber .Sm on .El The symbol may be prefixed with .Sq Ar symboltablename Ns :: (e.g., .Ql emulator::mach_msg_trap ) to specify other than kernel symbols. .It Ar number The radix is determined by the first two letters: .Ql 0x : hex, .Ql 0o : octal, .Ql 0t : decimal, otherwise, the value of .Va $radix is used. .It Li \&. .Va dot : the current address. .It Li + .Va next : the next address. .It Li .. The address of the start of the last line examined. Unlike .Va dot or .Va next , this is only changed by the .Ic examine or .Ic write command. .It Li ' The last address explicitly specified. .It Li $ Ns Ar variable The value of a register or variable. The name may be followed by a .Ql \&: and modifiers as described above with .Ar identifier . .It Ar expr Li # Ar expr A binary operator which rounds up the left hand side to the next multiple of right hand side. .It Li * Ns Ar expr Indirection. It may be followed by a ':' and modifiers as described above. .El .Sh SEE ALSO .Xr gdb 1 , .Xr nm 1 , .Xr sysctl.conf 5 , .Xr hangman 6 , .Xr kgdb 7 , .Xr crash 8 , .Xr sysctl 8 , .Xr extent 9 , .Xr pool 9 , .Xr uvm 9 .Sh HISTORY This kernel facility first appeared in the MACH 2 operating system developed by CMU. Hangman (which stands for "hangs maniacal analyzer") first appeared in .Ox 1.2 .