Cisco Systems Routers
Cisco Systems manufactures several routers with MIPS processors. Most of these routers are relatively modern, and sometimes are still prohibitively expensive on eBay or even via alternative channels of purchase. However, as many of these routers get decomissioned, they do appear on eBay at reasonable prices. A popular model with Linux/MIPS developers is the Cisco 3600-series.
Cisco has a very bland history when it comes to the class of CPUs in their hardware, often known for their (a)buse of m68k CPUs for their routers, as well as x86 CPUs for their firewalling and VPN solutions.
Cisco later switched to MIPS-based solutions from a variety of vendors, most notably IDT, PMC-Sierra and Broadcom, as well as some higher-end models being based on PowerPC processors.
|Model Number||CPU||CPU Clockspeed||Slot/Bus|
|1600||Motorola QUICC 68360||33MHz||PCMCIA WIC|
|1720||Motorola PowerQUICC MPC860||40MHz||PCMCIA WIC|
|1750||Motorola PowerQUICC MPC860||40MHz||PCMCIA WIC|
|2600||Motorola PowerQUICC MPC860||40MHz||NM|
|3620||IDT R4600||80MHz||NM (PCI)|
|3640||IDT R4600||100MHz||NM (PCI)|
|4000||Motorola 68030||40MHz||NP ?|
|4500M||IDT Orion RISC||100MHz||NP ?|
|4700M||IDT Orion RISC||133MHz||NP ?|
|7200 NPE100||R4700||150MHz||PA ?|
|7200 NPE150||R4700||150MHz||PA ?|
|7200 NPE175||RM5270||200MHz||PA ?|
|7200 NPE200||R5000||200MHz||PA ?|
|7200 NPE225||RM5271||225MHz||PA ?|
|7200 NPE300||RM7000||262MHz||PA ?|
|7200 NPE400||RM7000||350MHz||PA ?|
|7200 NSE-1||RM7000||263MHz||PA ?|
|7200 NPE-G1||BCM-1250||800MHz||PA ?|
|7200 NPE-G2||PowerPC 7448||1.67GHz||PA ?|
The Boot ROM
The Cisco Boot ROM firmware contains startup diagnostic code (ROM monitor, or ROMmon) as well as the boot loader for the Cisco Internetworking Operating System, or IOS for short.
During early boot, the code in the boot ROM performs a Power-on Self Test (POST) and, if all tests are passed, boot into IOS from the flash.
The Boot ROM CLI
In order to gain access to the monitor in the Boot ROM, send a break sequence to the device early on in the boot process. This can be done with a C-a f (Ctrl+a f) in Minicom, for example.
Recently boot ROMs have had an undocumented
priv command. This command then can be used to gain access to several additional commands, including a debugger, disassembler and additional hardware tests. To use this command, a secret password is required, known only to Cisco representatives. However, in recent times this has been cracked and the password for many models of Cisco routers can be calculated using the tool at http://ers.pp.ru/cisco/priv.html.
Boot ROM maintains certain system configuration parameters in environment variables. For example, the $MONRC can contain a starup command sequence, $PS1 contains the command prompt, and so forth. As well, the BootROM supports basic command aliasing. Both the environment variables and the alias table are stored in NVRAM so that their values persist, even when power is off.
The Boot ROM API
The Boot ROM API provides some simple APIs for IOS (for example, putchar and version information commands). Unlike many other firmwares in the world of MIPS, the
syscall opcode is used to call these firmware APIs. Note that register a0 must contain the syscall number.
A sample "Hello, world!" program can be found here.
IOS executables are shipped in a raw binary format (known as a .bin file to many). For MIPS-based devices, this is just conventional MIPS Big-Endian ELF, however Cisco does play a dirty trick in using a non-standard
e_machine value in the ELF header. This seems to be based on the router model. For example, the Cisco 3600 routers have an
e_machine value of
In order to alter the
e_machine value, a recent version of objcopy can be used with the switch
--alt-machine-code 0x1e (in this example for a Cisco 3600 series machine).
As well, there are limitations placed on the binary format due to the behaviour of the software in the Boot ROM. Boot ROM cannot load multiple ELF program headers. Thus, to work around this problem, toolchains must be built with --target=mips-elf.
Finally, all symbol tables must be removed.
The Boot ROM can load and execute a block of executable code (such as IOS) from various internal locations: the internal FLASH module(s), a PCMCIA Linear Flash card and (unofficially) TFTP. More recent models also support PCMCIA IDE, CompactFlash and USB flash. Also the Boot ROM supports compressed images (.gz) with embedded helper and text files that contain a command sequence (like a shell script). Nowadays IOS distributions are compressed with ZIP and have a built-in ELF ZIP decompressor "piggybacked" on top, since ZIP provides better compression.
Cisco 3600R4700 MIPS CPUs. The system controller is a Galileo GT-64010.
The Cisco 3600-series routers were designed with limited expandability in mind. As such, the Cisco 3620 has two expansion slots; the 3640 has four (as is reflected in the size of the units). These expansion modules sit on the NM expansion bus, which is essentially a modified PCI 2.1 bus with some proprietary extensions relating to EEPROM identication and what Cisco calls OIR, or Online Insertion and Removal.
Deeper technical information on the Cisco 3600-series can be found at http://www.cisco.com/warp/public/63/36xx-arch.pdf.
Presently Linux does not boot on the Cisco 3600 series in any usable form. However, a PROM library has been established and most PROM calls of importance have been reverse engineered. As well, efforts to make Linux run on the 3600 seriers are well underway and could very well be bootable soon.