System/161 MIPS Processor

The 32-bit MIPS is the simplest "real" 32-bit processor for which development tools are readily available; furthermore, the MIPS architecture is already widely used for teaching in various contexts. This makes it a natural choice for System/161.

The specific dialect of MIPS processor found in System/161, which for a lack of a better term we'll refer to as MIPS-161, is essentially a cache-coherent r3000, or MIPS-I.

User Mode

In user mode, the MIPS-161 behaves the same as any other 32-bit MIPS. All user instructions are fully interlocked and there are no pipeline hazards. All MIPS-I instructions are supported. MIPS-II and higher instructions are not supported, except for the ll and sc instructions used for multiprocessor synchronization. Please consult your favorite MIPS reference for further details.

Kernel Mode

In kernel mode, the MIPS-161 is mostly a MIPS-I, with a few differences and a few extensions borrowed from later MIPS versions. For completeness, the following sections define the complete kernel mode interface.

Kernel Instructions

The WAIT instruction has been borrowed from MIPS-II. This operation puts the processor into a low-power state and suspends execution until some external event occurs, such as an interrupt. Since the exact behavior of WAIT is not clearly specified anywhere I could find, the MIPS-161 behavior is as follows:

WAIT should be executed with interrupts disabled or masked in the processor.
WAIT will complete when any of the processor interrupt pins are activated.
The interrupt is not serviced until it is unmasked or re-enabled.

Regarding the TLBR, TLBWR, TLBWI, and TLBP instructions, see the MMU section below. Regarding the RFE instruction, see the trap handling section, also below. Regarding the MFC0 and MTC0 instructions, see the next section.

Kernel Registers

The MIPS-161 has 10 supervisor registers in coprocessor 0. These may be accessed with the MFC0 (move from coprocessor 0) and MTC0 instruction, as follows:

	mfc0 $4, $12

loads the contents of supervisor register 12 (STATUS) into general-purpose register 4 (a0), and

	mtc0 $4, $12

does the reverse.

The supervisor registers are:

INDEX ($0)
RANDOM ($1)
TLBLO ($2)
CONTEXT ($4)
BADVADDR ($8)
TLBHI ($10)
STATUS ($12)
CAUSE ($13)
EPC ($14)
PRID ($15)

with the following bit patterns:

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0	Bits
P	0																	SLOT						0									INDEX
0																		SLOT						0									RANDOM
PPAGE																				N	D	V	G	0									TLBLO
PTBASE											VSHIFT																			0			CONTEXT
BADVADDR																																	BADVADDR
VPAGE																				ASID						0							TLBHI
d	c	b	a	0					B	T	E	M	Z	S	I	H	H	H	H	H	H	F	F	0		KUo	IEo	KUp	IEp	KUc	IEc		STATUS
BD	0	CE		0												H	H	H	H	H	H	F	F	0		EXC				0			CAUSE
EPC																																	EPC
0																PRID																	PRID

INDEX

P -- probe failure. Set if the TLBP instruction fails to find a matching entry in the TLB.
SLOT -- TLB entry index, 0-63. This field is set by the TLBP instruction; it is also used by the TLBWI instruction.

RANDOM

SLOT -- TLB entry index, 0-63. This field is used by the TLBWR instruction and is incremented on every clock cycle. See below.

TLBLO

PPAGE -- physical page number for this TLB entry.
N -- noncached; if set, accesses via this TLB entry will be uncached.
D -- dirty; if set, write accesses via this TLB entry will be permitted; otherwise they will trap. See below.
V -- valid; if set, accesses via this TLB entry will be permitted; otherwise they will trap. See below.
G -- global; if set, the ASID field will be ignored when matching this TLB entry.

CONTEXT

PTBASE -- base address of page table. See below.
VSHIFT -- pre-masked-and-shifted BADVADDR for indexing page table. See below.

BADVADDR

BADVADDR -- failing virtual address on MMU exception.

TLBHI

VPAGE -- virtual page number for this TLB entry.
ASID -- address space ID for this TLB entry.

STATUS

abcdTEMZSI -- cache control and other related bits. Set to 0.
B -- boot flag. If set, exceptions go to the boot ROM instead of the kernel.
H -- hardware interrupt enable bit, lines 0-5.
F -- software interrupt enable bit, lines 0-1.
KU -- 1 if user mode, 0 if kernel mode.
IE -- 1 if interrupts enabled, 0 if disabled. See below regarding o/p/c.

CAUSE

BD -- 1 if exception occurred in a branch delay slot.
CE -- coprocessor number for exception, if any.
H -- hardware interrupt state, 1=active, lines 0-5.
F -- software interrupt state, 1=active, lines 0-1.
EXC -- exception code.

EPC

EPC -- saved program counter from exception.

PRID

PRID -- processor ID code. The MIPS-161 sets this to 0xbeef.

Trap Handling

When an exception occurs, the following things happen:

The PC where the exception occurred is loaded into the EPC register.

If this was in a branch delay slot, the EPC register is set to the address of the branch (that is, 4 is subtracted) and the BD flag in the CAUSE register is set. Software need not examine the BD flag unless the exact address of the faulting instruction is wanted, e.g. for disassembly and analysis.

The EXC field of the CAUSE register is set to reflect what happened. The exception codes are listed below.

For coprocessor-related exceptions the CE field of the CAUSE register is set.

For interrupts the H and F bits of the CAUSE register are set to reflect the interrupt(s) that are active.

For MMU exceptions the BADVADDR register is loaded with the failing address. A masked and shifted form suitable for indexing a page table is placed in the VSHIFT field of the CONTEXT register.

The bottom six bits of the STATUS register are shifted left by two. The "o" (old) bits are lost; the "p" (previous) bits become the old bits; the "c" (current) bits become the previous bits; and the current bits are set to 0. This disables interrupts and puts the processor in kernel mode.

Execution continues from one of five hardwired addresses according to what happened and the setting of the B (boot) bit in the STATUS register.

The exception handler addresses are:

B Trap Address

1 General 0xbfc0 0180

1 UTLB 0xbfc0 0100

1 Reset 0xbfc0 0000

0 General 0x8000 0080

0 UTLB 0x8000 0000

A UTLB exception is a TLB miss that occurs against the user address space (0x0000 0000 - 0x7fff ffff) and occurs because no matching TLB entry was found. Other TLB exceptions go through the General vector. This allows a fast-path TLB refill handler. See below.

To return from an exception, one executes the following sequence:

	jr k0
	rfe

where the k0 register has been loaded with the desired exception return address, either the value previously retrieved from the EPC register or some other address chosen by software. The RFE instruction is not a jump; it occurs in the delay slot of a jump. It shifts the six bottom bits of the status register right by two, undoing the shift done at exception entry time. This returns the processor to whatever interrupt state and mode (user/kernel) it was in when the exception occurred.

Because there are three pairs of state bits, the processor can take two nested exceptions without losing state, if one is careful. This is to facilitate the fast-path TLB refill handler. See below.

The two soft interrupt lines can be activated by writing to the CAUSE register.

The exceptions:

IRQ (0) -- Interrupt
MOD (1) -- "Modify", TLB read-only fault
TLBL (2) -- TLB miss on load
TLBS (3) -- TLB miss on store
ADEL (4) -- Address error on load
ADES (5) -- Address error on store
IBE (6) -- Bus error on instruction fetch
DBE (7) -- Bus error on data access
SYS (8) -- System call
BP (9) -- Breakpoint instruction
RI (10) -- Reserved (illegal) instruction
CPU (11) -- Coprocessor unusable
OVF (12) -- Integer overflow
13-15 -- Reserved

The IBE and DBE exceptions are not MMU exceptions and do not set BADVADDR.

MMU

The MMU is the MIPS-I MMU, with a 64-entry TLB where each entry maps one 4K virtual page to one 4K physical page. The paired pages setup of later MIPS processors is not present, and there is no support for superpages.

The processor's virtual address space is divided into four segments:

Name Description

kseg2 Supervisor mode only; TLB-mapped, cacheable

kseg1 Supervisor mode only; direct-mapped, uncached

kseg0 Supervisor mode only; direct-mapped, cached

kuseg User and supervisor mode; TLB-mapped, cacheable

The mapped segments are mapped via a translation lookaside buffer (TLB) with software refill. The direct-mapped segments are mapped (without use of the TLB) both to the first 512 megabytes of the physical memory space. Typically the kernel lives in kseg0, hardware devices are accessed through kseg1, and user-mode programs are run in kuseg.

There are four MMU-related instructions:

TLBR uses the SLOT field of the INDEX register to choose a TLB entry, and reads it into the TLBLO and TLBHI registers.
TLBWI uses the SLOT field of the INDEX register to choose a TLB entry, and writes the TLBLO and TLBHI registers into it.
TLBWR uses the SLOT field of the RANDOM register to choose a TLB entry, and writes the TLBLO and TLBHI registers into it.
TLBP searches the TLB for an entry matching the TLBHI register, and sets the SLOT field of the INDEX register to its number, unless no entry was found, in which case the P field of the INDEX register is set to -1. (This makes the value of the INDEX register negative, which is easily tested.)

The INDEX field of the RANDOM register ranges from 8 to 63; it is incremented on every instruction executed, which is not very random but apparently adequate for the purpose, which is to fill the TLB rapidly and effectively. Entries 0 through 7 of the TLB are never touched by TLBWR and can be used for reserved or special mappings.

The processor is build to support a fast-path TLB refill handler, which is invoked via the UTLB exception vector (see above). The idea is that the OS maintains page tables in virtual memory using the kseg2 region (see above) and loads the base address of the page table into the PTBASE field of the CONTEXT register. Each page table entry is a 4-byte quantity suitable for loading directly into the TLBLO register; 1024 of these fit on a 4K page, so each page table page maps 4MB and it takes 512 pages, or 2MB of virtual space, to map the whole 2GB user address space. (Since these are placed in virtual memory, only the page table pages that are used need be materialized.) With this setup, the UTLB exception handler can then read the CONTEXT register and use the resulting value to load directly from the page table. If this fails because that section of the page table is not materialized, a second (non-UTLB) exception occurs. Careful register usage and the three-deep nesting of the bottom part of the STATUS register allows the general-purpose exception handler to recover from this condition and proceed as desired. On success, the UTLB handler can then unconditionally write the PTE it got into the TLB. If the V (valid) bit is not set, on return from the UTLB handler another exception will occur; however, because a matching (though not valid) TLB entry exists, this will not be a UTLB exception, and the general exception handler will get control and can schedule pagein or whatever.

There are a number of possible other ways to use the UTLB handler, of course. One simple way is to just have it jump to the general-purpose exception handler.

As noted above, the V (valid) bit does not prevent a TLB entry from being "matching". A TLB entry is matching if both of the following are true:

The VPAGE of the entry matches the virtual address being translated, and
either the ASID of the entry matches the ASID currently loaded into the TLBHI register, or the entry has the G (global) flag set.

One must never load the TLB in such a fashion that two (or more entries) can match the same virtual address. If this happens, the processor shuts down and is unrecoverable except by hard reset. Since there is no way to prevent entries from matching, one should clear the TLB by loading each entry with a distinct VPAGE, and use VPAGEs from the kseg0 or kseg1 regions that will never be presented to the MMU for translation. To reset the TLB at startup, since it is not cleared by processor reset, one should use a second, potentially larger, set of distinct VPAGEs and check that each is not already present before loading it.

There is no way to tell if a TLB entry has been used, or how recently it has been used. Nor is there a direct way to tell if a TLB entry has been used for writing. The D ("dirty") bit can be used for this purpose with software support, as follows:

When the TLB entry is first loaded, clear the D bit. This will make the MMU treat the translation as read-only.
Upon a write, a MOD exception will occur. If writing through this translation is legal, set the D bit in the TLB and anywhere else in the virtual memory system data structures that might be desired. The page is now dirty, and the processor will write to it freely.
When the page is written back or otherwise cleaned, or on a periodic basis to measure usage over time, flush the TLB entry or reload it with the D bit clear again.

The MMU exceptions are as follows:

MOD -- attempt to write through a TLB entry whose D bit is not set. Software should respond either by making the TLB entry writable or treating the condition as a failure.
TLBL -- TLB miss (entry not present, or entry not valid) on a data read. If the address was in kuseg and the entry was not present, the UTLB handler is invoked. Otherwise the general exception handler is invoked. The failing address is placed in BADVADDR, and as noted above in the VSHIFT field of the CONTEXT register. The VPAGE field of the TLBHI register is also loaded. Software should respond by loading a matching and valid translation, or treating the condition as a failure.
TLBS -- TLB miss on store. Otherwise the same as TLBL.
ADEL -- address error on load. An address error is either an unaligned access or an attempt to access kernel memory while in user mode. Software should respond by treating the condition as a failure.
ADES -- same as ADEL, but for stores.

Cache Control

The MIPS-I has a remarkably painful cache and cache control architecture. While the MIPS-161 exhibits the same cache control bits in the STATUS register, it is in fact cache-coherent and there is no need to flush, examine, or otherwise touch the cache subsystem. In fact, doing any of these things in the MIPS-I fashion will result in undefined behavior.

Since normal MIPS processors have split instruction and data caches, and future System/161 releases may include more cache handling, it is recommended that all necessary flushes of the instruction cache be included and stubbed out.

Startup

On CPU reset execution begins from the Reset vector defined above. The processor starts out in an almost completely undefined state. The cache is in an undefined state (except on the MIPS-161 this does not matter...), the TLB is in an undefined state, and the contents of the general and kernel-mode registers are all undefined, except as follows:

The B (boot) flag of the STATUS register is set to 1, so exceptions will go to the boot exception vectors in kseg1 instead of the kernel exception vectors in kseg0.
The KUc and IEc bits of the STATUS register are set to 0, so the processor is in kernel mode and interrupts are disabled.

The code at the Reset vector must in general sort out the processor state before it can do anything else.

In System/161, the boot ROM takes care of these issues and loads a kernel as described in the LAMEbus documentation. However, the state guaranteed by the boot ROM is only slightly more flexible: the boot ROM guarantees that the cache is in a workable state, and it provides a stack and an argument string in the a0 register. The TLB is still in an undefined state and the contents of other general and kernel-mode registers and register fields are still undefined.

Identifying the Processor

Currently, code that knows it is running on System/161 may assume it has a MIPS-161 and proceed accordingly.

Code that wants to run unchanged on a variety of MIPS platforms without a System/161-specific startup wedge is likely to run into problems: there is no safe way to identify that one is running on System/161 as such, and distinguishing the MIPS-161 from an arbitrarily chosen MIPS-I is likely to be problematic. The MIPS-161 sets the PRID register to 0x0000 beef; however, my understanding is that the contents of the PRID register for early MIPS models (where the upper 16 bits are defined as 0) cannot even be used reliably to distinguish real deployed hardware. It might be possible to distinguish the MIPS-161 based on its cache (non-)behavior; however, this is probably dangerous and not recommended.