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## 11 CPU Configuration

This chapter discusses how to set up GDB debug targets for CPUs. You can also access these targets without GDB (see Architecture and Core Commands, and Target State handling) and through various kinds of NAND and NOR flash commands. If you have multiple CPUs you can have multiple such targets.

We'll start by looking at how to examine the targets you have, then look at how to add one more target and how to configure it.

### 11.1 Target List

All targets that have been set up are part of a list, where each member has a name. That name should normally be the same as the TAP name. You can display the list with the `targets` (plural!) command. This display often has only one CPU; here's what it might look like with more than one:

        TargetName         Type       Endian TapName            State
    --  ------------------ ---------- ------ ------------------ ------------
     0* at91rm9200.cpu     arm920t    little at91rm9200.cpu     running
     1  MyTarget           cortex_m3  little mychip.foo         tap-disabled

One member of that list is the current target, which is implicitly referenced by many commands. It's the one marked with a `*` near the target name. In particular, memory addresses often refer to the address space seen by that current target. Commands like `mdw` (memory display words) and `flash erase_address` (erase NOR flash blocks) are examples; and there are many more.

Several commands let you examine the list of targets:

— Command: target count

> Note: target numbers are deprecated; don't use them. They will be removed shortly after August 2010, including this command. Iterate target using `target names`, not by counting.

> Returns the number of targets, N. The highest numbered target is N - 1.

>               set c [target count]
>               for { set x 0 } { $x < $c } { incr x } {
>                   # Assuming you have created this function
>                   print_target_details $x
>               }

— Command: target current

> Returns the name of the current target.

— Command: target names

> Lists the names of all current targets in the list.

>               foreach t [target names] {
>                   puts [format "Target: %s\n" $t]
>               }

— Command: target number number

> Note: target numbers are deprecated; don't use them. They will be removed shortly after August 2010, including this command.

> The list of targets is numbered starting at zero. This command returns the name of the target at index number.

>               set thename [target number $x]
>               puts [format "Target %d is: %s\n" $x $thename]

— Command: targets [name]

> Note: the name of this command is plural. Other target command names are singular.

> With no parameter, this command displays a table of all known targets in a user friendly form.

> With a parameter, this command sets the current target to the given target with the given name; this is only relevant on boards which have more than one target.

### 11.2 Target CPU Types and Variants

Each target has a CPU type, as shown in the output of the `targets` command. You need to specify that type when calling `target create`. The CPU type indicates more than just the instruction set. It also indicates how that instruction set is implemented, what kind of debug support it integrates, whether it has an MMU (and if so, what kind), what core-specific commands may be available (see Architecture and Core Commands), and more.

For some CPU types, OpenOCD also defines variants which indicate differences that affect their handling. For example, a particular implementation bug might need to be worked around in some chip versions.

It's easy to see what target types are supported, since there's a command to list them. However, there is currently no way to list what target variants are supported (other than by reading the OpenOCD source code).

— Command: target types

> Lists all supported target types. At this writing, the supported CPU types and variants are:

> - `arm11` – this is a generation of ARMv6 cores
> - `arm720t` – this is an ARMv4 core with an MMU
> - `arm7tdmi` – this is an ARMv4 core
> - `arm920t` – this is an ARMv4 core with an MMU
> - `arm926ejs` – this is an ARMv5 core with an MMU
> - `arm966e` – this is an ARMv5 core
> - `arm9tdmi` – this is an ARMv4 core
> - `avr` – implements Atmel's 8-bit AVR instruction set. (Support for this is preliminary and incomplete.)
> - `cortex_a8` – this is an ARMv7 core with an MMU
> - `cortex_m3` – this is an ARMv7 core, supporting only the compact Thumb2 instruction set. It supports one variant:
>   - `lm3s` ... Use this when debugging older Stellaris LM3S targets. This will cause OpenOCD to use a software reset rather than asserting SRST, to avoid a issue with clearing the debug registers. This is fixed in Fury Rev B, DustDevil Rev B, Tempest; these revisions will be detected and the normal reset behaviour used.
> - `dragonite` – resembles arm966e
> - `dsp563xx` – implements Freescale's 24-bit DSP. (Support for this is still incomplete.)
> - `fa526` – resembles arm920 (w/o Thumb)
> - `feroceon` – resembles arm926
> - `mips_m4k` – a MIPS core. This supports one variant:
> - `xscale` – this is actually an architecture, not a CPU type. It is based on the ARMv5 architecture. There are several variants defined:
>   - `ixp42x`, `ixp45x`, `ixp46x`, `pxa27x` ... instruction register length is 7 bits
>   - `pxa250`, `pxa255`, `pxa26x` ... instruction register length is 5 bits
>   - `pxa3xx` ... instruction register length is 11 bits

To avoid being confused by the variety of ARM based cores, remember this key point: ARM is a technology licencing company. (See: http://www.arm.com.) The CPU name used by OpenOCD will reflect the CPU design that was licenced, not a vendor brand which incorporates that design. Name prefixes like arm7, arm9, arm11, and cortex reflect design generations; while names like ARMv4, ARMv5, ARMv6, and ARMv7 reflect an architecture version implemented by a CPU design.

### 11.3 Target Configuration

Before creating a “target”, you must have added its TAP to the scan chain. When you've added that TAP, you will have a `dotted.name` which is used to set up the CPU support. The chip-specific configuration file will normally configure its CPU(s) right after it adds all of the chip's TAPs to the scan chain.

Although you can set up a target in one step, it's often clearer if you use shorter commands and do it in two steps: create it, then configure optional parts. All operations on the target after it's created will use a new command, created as part of target creation.

The two main things to configure after target creation are a work area, which usually has target-specific defaults even if the board setup code overrides them later; and event handlers (see Target Events), which tend to be much more board-specific. The key steps you use might look something like this

         target create MyTarget cortex_m3 -chain-position mychip.cpu
         $MyTarget configure -work-area-phys 0x08000 -work-area-size 8096
         $MyTarget configure -event reset-deassert-pre { jtag_rclk 5 }
         $MyTarget configure -event reset-init { myboard_reinit }

You should specify a working area if you can; typically it uses some on-chip SRAM. Such a working area can speed up many things, including bulk writes to target memory; flash operations like checking to see if memory needs to be erased; GDB memory checksumming; and more.

> Warning: On more complex chips, the work area can become inaccessible when application code (such as an operating system) enables or disables the MMU. For example, the particular MMU context used to acess the virtual address will probably matter ... and that context might not have easy access to other addresses needed. At this writing, OpenOCD doesn't have much MMU intelligence.

It's often very useful to define a `reset-init` event handler. For systems that are normally used with a boot loader, common tasks include updating clocks and initializing memory controllers. That may be needed to let you write the boot loader into flash, in order to “de-brick” your board; or to load programs into external DDR memory without having run the boot loader.

— Command: target create target_name type configparams...

> This command creates a GDB debug target that refers to a specific JTAG tap. It enters that target into a list, and creates a new command (`target_name`) which is used for various purposes including additional configuration.

> - target_name ... is the name of the debug target. By convention this should be the same as the dotted.name of the TAP associated with this target, which must be specified here using the `-chain-position` dotted.name configparam.

>   This name is also used to create the target object command, referred to here as `$target_name`, and in other places the target needs to be identified.

> - type ... specifies the target type. See target types.
> - configparams ... all parameters accepted by `$target_name configure` are permitted. If the target is big-endian, set it here with `-endian big`. If the variant matters, set it here with `-variant`.

>   You must set the `-chain-position` dotted.name here.

— Command: $target_name configure configparams...

> The options accepted by this command may also be specified as parameters to `target create`. Their values can later be queried one at a time by using the `$target_name cget` command.

> Warning: changing some of these after setup is dangerous. For example, moving a target from one TAP to another; and changing its endianness or variant.

> - `-chain-position` dotted.name – names the TAP used to access this target.
> - `-endian` (`big`|`little`) – specifies whether the CPU uses big or little endian conventions
> - `-event` event_name event_body – See Target Events. Note that this updates a list of named event handlers. Calling this twice with two different event names assigns two different handlers, but calling it twice with the same event name assigns only one handler.
> - `-variant` name – specifies a variant of the target, which OpenOCD needs to know about.
> - `-work-area-backup` (`0`|`1`) – says whether the work area gets backed up; by default, it is not backed up. When possible, use a working_area that doesn't need to be backed up, since performing a backup slows down operations. For example, the beginning of an SRAM block is likely to be used by most build systems, but the end is often unused.
> - `-work-area-size` size – specify work are size, in bytes. The same size applies regardless of whether its physical or virtual address is being used.
> - `-work-area-phys` address – set the work area base address to be used when no MMU is active.
> - `-work-area-virt` address – set the work area base address to be used when an MMU is active. Do not specify a value for this except on targets with an MMU. The value should normally correspond to a static mapping for the `-work-area-phys` address, set up by the current operating system.

### 11.4 Other $target_name Commands

The Tcl/Tk language has the concept of object commands, and OpenOCD adopts that same model for targets.

A good Tk example is a on screen button. Once a button is created a button has a name (a path in Tk terms) and that name is useable as a first class command. For example in Tk, one can create a button and later configure it like this:

         # Create
         button .foobar -background red -command { foo }
         # Modify
         .foobar configure -foreground blue
         # Query
         set x [.foobar cget -background]
         # Report
         puts [format "The button is %s" $x]

In OpenOCD's terms, the “target” is an object just like a Tcl/Tk button, and its object commands are invoked the same way.

         str912.cpu    mww 0x1234 0x42
         omap3530.cpu  mww 0x5555 123

The commands supported by OpenOCD target objects are:

— Command: $target_name arp_examine
— Command: $target_name arp_halt
— Command: $target_name arp_poll
— Command: $target_name arp_reset
— Command: $target_name arp_waitstate

> Internal OpenOCD scripts (most notably `startup.tcl`) use these to deal with specific reset cases. They are not otherwise documented here.

— Command: $target_name array2mem arrayname width address count
— Command: $target_name mem2array arrayname width address count

> These provide an efficient script-oriented interface to memory. The `array2mem` primitive writes bytes, halfwords, or words; while `mem2array` reads them. In both cases, the TCL side uses an array, and the target side uses raw memory.

> The efficiency comes from enabling the use of bulk JTAG data transfer operations. The script orientation comes from working with data values that are packaged for use by TCL scripts; `mdw` type primitives only print data they retrieve, and neither store nor return those values.

> - arrayname ... is the name of an array variable
> - width ... is 8/16/32 - indicating the memory access size
> - address ... is the target memory address
> - count ... is the number of elements to process

— Command: $target_name cget queryparm

> Each configuration parameter accepted by `$target_name configure` can be individually queried, to return its current value. The queryparm is a parameter name accepted by that command, such as `-work-area-phys`. There are a few special cases:

> - `-event` event_name – returns the handler for the event named event_name. This is a special case because setting a handler requires two parameters.
> - `-type` – returns the target type. This is a special case because this is set using `target create` and can't be changed using `$target_name configure`.

> For example, if you wanted to summarize information about all the targets you might use something like this:

>               foreach name [target names] {
>                   set y [$name cget -endian]
>                   set z [$name cget -type]
>                   puts [format "Chip %d is %s, Endian: %s, type: %s" \
>                                $x $name $y $z]
>               }

— Command: $target_name curstate

> Displays the current target state: `debug-running`, `halted`, `reset`, `running`, or `unknown`. (Also, see Event Polling.)

— Command: $target_name eventlist

> Displays a table listing all event handlers currently associated with this target. See Target Events.

— Command: $target_name invoke-event event_name

> Invokes the handler for the event named event_name. (This is primarily intended for use by OpenOCD framework code, for example by the reset code in `startup.tcl`.)

— Command: $target_name mdw addr [count]
— Command: $target_name mdh addr [count]
— Command: $target_name mdb addr [count]

> Display contents of address addr, as 32-bit words (`mdw`), 16-bit halfwords (`mdh`), or 8-bit bytes (`mdb`). If count is specified, displays that many units. (If you want to manipulate the data instead of displaying it, see the `mem2array` primitives.)

— Command: $target_name mww addr word
— Command: $target_name mwh addr halfword
— Command: $target_name mwb addr byte

> Writes the specified word (32 bits), halfword (16 bits), or byte (8-bit) pattern, at the specified address addr.

### 11.5 Target Events

At various times, certain things can happen, or you want them to happen. For example:

- What should happen when GDB connects? Should your target reset?
- When GDB tries to flash the target, do you need to enable the flash via a special command?
- Is using SRST appropriate (and possible) on your system? Or instead of that, do you need to issue JTAG commands to trigger reset? SRST usually resets everything on the scan chain, which can be inappropriate.
- During reset, do you need to write to certain memory locations to set up system clocks or to reconfigure the SDRAM? How about configuring the watchdog timer, or other peripherals, to stop running while you hold the core stopped for debugging?

All of the above items can be addressed by target event handlers. These are set up by `$target_name configure -event` or `target create ... -event`.

The programmer's model matches the `-command` option used in Tcl/Tk buttons and events. The two examples below act the same, but one creates and invokes a small procedure while the other inlines it.

         proc my_attach_proc { } {
             echo "Reset..."
             reset halt
         }
         mychip.cpu configure -event gdb-attach my_attach_proc
         mychip.cpu configure -event gdb-attach {
             echo "Reset..."
             # To make flash probe and gdb load to flash work we need a reset init.
             reset init
         }

The following target events are defined:

- debug-halted
  The target has halted for debug reasons (i.e.: breakpoint)
- debug-resumed
  The target has resumed (i.e.: gdb said run)
- early-halted
  Occurs early in the halt process
- gdb-attach
  When GDB connects. This is before any communication with the target, so this can be used to set up the target so it is possible to probe flash. Probing flash is necessary during gdb connect if gdb load is to write the image to flash. Another use of the flash memory map is for GDB to automatically hardware/software breakpoints depending on whether the breakpoint is in RAM or read only memory.
- gdb-detach
  When GDB disconnects
- gdb-end
  When the target has halted and GDB is not doing anything (see early halt)
- gdb-flash-erase-start
  Before the GDB flash process tries to erase the flash
- gdb-flash-erase-end
  After the GDB flash process has finished erasing the flash
- gdb-flash-write-start
  Before GDB writes to the flash
- gdb-flash-write-end
  After GDB writes to the flash
- gdb-start
  Before the target steps, gdb is trying to start/resume the target
- halted
  The target has halted
- reset-assert-pre
  Issued as part of `reset` processing after `reset_init` was triggered but before either SRST alone is re-asserted on the scan chain, or `reset-assert` is triggered.
- reset-assert
  Issued as part of `reset` processing after `reset-assert-pre` was triggered. When such a handler is present, cores which support this event will use it instead of asserting SRST. This support is essential for debugging with JTAG interfaces which don't include an SRST line (JTAG doesn't require SRST), and for selective reset on scan chains that have multiple targets.
- reset-assert-post
  Issued as part of `reset` processing after `reset-assert` has been triggered. or the target asserted SRST on the entire scan chain.
- reset-deassert-pre
  Issued as part of `reset` processing after `reset-assert-post` has been triggered.
- reset-deassert-post
  Issued as part of `reset` processing after `reset-deassert-pre` has been triggered and (if the target is using it) after SRST has been released on the scan chain.
- reset-end
  Issued as the final step in `reset` processing.
- reset-init
  Used by reset init command for board-specific initialization. This event fires after reset-deassert-post.

  This is where you would configure PLLs and clocking, set up DRAM so you can download programs that don't fit in on-chip SRAM, set up pin multiplexing, and so on. (You may be able to switch to a fast JTAG clock rate here, after the target clocks are fully set up.)

- reset-start
  Issued as part of `reset` processing before `reset_init` is called.

  This is the most robust place to use `jtag_rclk` or `adapter_khz` to switch to a low JTAG clock rate, when reset disables PLLs needed to use a fast clock.

- resume-start
  Before any target is resumed
- resume-end
  After all targets have resumed
- resume-ok
  Success
- resumed
  Target has resumed