Authored by Jerzy Tarasiuk
Posted on 25 May 1995 17:37:03 +0200

Real and Protected Modes.

Beginning from 80286, Intel CPUs have ability to work in Protected Mode
(older CPUs have Real Mode only). For compatibility reasons, all CPUs
start in Real Mode after reset. Below are presented main differences
between Real Mode and Protected modes for Intel CPUs. Note there are:
Real Mode, Protected Mode, Virtual 8086 Mode (they will be frequently
called RM, PM, VM86, respectively; also 286+(386+) will mean Intel
80286(80386) or better).

There are some differences between these modes in memory addressing
(PM can address all memory, while RM can't unless it is set in PM on
386+, and VM86 cannot unless using PM supporting it to remap memory
- this way EMM386 works); instruction set (some instruction are not
allowed in RM), privileges (something can be forbidden in PM for less
privileged code, many operations are forbidden in VM86), interrupt
handling. PM supports multitasking, PM can run tasks in VM86 (the
VM86 cannot function alone, must have PM code supporting it; it works
similarly 8086 CPU with few enhancements except interrupt servicing
which goes through PM). PM cannot store data to code segment (unless
by aliasing; MOV CS:[BX],AX is illegal in PM). VM86 and PM on 386+ can
have selective I/O port access restrictions (some ports can be accessed
without causing exception and other can't).


Memory addressing and Paging.

In any mode, opcode defines some offset and segment of referenced memory
address, e.g. mov ax,es:[bx+si+1] - segment es, offset bx+si+1, push si
- segment ss, offset sp-2, opcode itself is referenced by segment cs and
offset ip; the address is translated to Linear Address by adding the
offset to base of the segment and the Linear Address is then translated
to Physical Address which is outputed by CPU on its address pins.

In RM or VM86, the base is segment*10h; in PM the base is taken from
descriptor table (LDT or GDT) and can have any value.
The value in segment register is called "selector" and its bits 15-3
specify offset in LDT or GDT (the offset is multiply of 8), bit 2 is 0
for GDT, 1 for LDT, bits 1-0 specify RPL (Requested Privilege Level).

Unless Paging (possible in PM and VM86, on 386+ only) is enabled,
Physical Address = Linear. With Paging, low 12 bits of Linear Address
go to Physical, other are used as index to two-level page tables
(first bits 31-22 select page directory, then bits 21-12 select page).

Paging can also restrict access to some pages (in a way non-privileged
code can read it only or has no access at all), or define non-present
pages which have assigned physical addresses and put in memory in a way
transparent to program when access to their Linear Address is attempted.
Note Linear Address space is 4GB on 386+, and probably no system has so
much physical memory: Paging makes system able to simulate it has.

Segment has also limit. Initially, the limit is 0FFFFh for all segment
registers and cannot be changed in RM or VM86. In PM it is loaded from
LDT or GDT when segment register is loaded. On 286 in PM the limit can
be up to 0FFFFh, on 386+ in PM it can be up to 0FFFFFFFFh.
Also, PM allows "expand down" segments which allow access from address
limit+1 to maximum possible value of limit (depend on segment type).


Privilege Levels and Rules.

In RM, CPU has full privileges. In PM and VM86, they can be restricted.
This reduces possibility of making disasters by bad code.

Base rules: cannot access more privileged data or call less privileged
code than own privilege (although can return to less privileged code).
Additional: call to more privileged code cannot use any target address
caller wants, it can use addresses specified by system only; call to
more privileged code must change stack to make sure enough stack space
is available for called code (so caller cannot cause crash in it).

There are 4 levels: level 0 is full privilege (except Debug Registers,
which can be protected from access even from level 0; some instructions
are reserved for level 0 only), the bigger level the less privileges
are. Few terms used for Privilege Levels: CPL - Current PL, DPL -
Descriptor PL, RPL - Requested PL (in selector), IOPL (in flags) -
max CPL allowing I/O sensitive opcodes (CLI, STI, PUSHF, POPF,...).

Unless accessing Conforming Code segment, privilege rules require
max(CPL,RPL)<=DPL. To execute code (by FAR CALL or JMP) need DPL<=CPL
(note unless it is Conforming, must be DPL=CPL and RPL<=CPL) - cannot
call less privileged procedure, for example. To transfer control to
code with less PL (more privileged), must CALL via call gate (in such
a case, need max(CPL,RPL)<=gate_DPL, but for code the gate refers to
may be code_DPL<gate_DPL; the gate is entry in GDT or LDT; privilege
rules require also target_code_DPL <= CPL for CALL, = for JMP), this
also requires TR to point to valid TSS because it switches stack: old
SS:[E]SP are pushed on new stack, then parameters (as defined in call
gate) are pushed, finally CS:[E]IP are pushed. On return from the call
CPU detects RPL of CS on stack > CPL and switches stack back (if =, no
stack switch, < inhibited by privilege rules), for proper functioning
parameter counts on RET and in call gate must match. For stack segment
DPL must be equal CPL (so in more privileged mode no crash is possible
due to incorrect stack setting in less privileged, and in the less
privileged there is no access to more privileged mode stack).

The RPL is for system to block possibility to pass a pointer from user
code which is invalid in user mode and valid in system: system uses RPL
as for user code and gets access violation error in such a case.
It can be done using ARPL opcode which adjusts RPL for a selector, and
sets ZF if changed (to inform OS invalid access might be attempted).
OS uses it to set RPL of the pointer to CPL of the application code.

It is possible to check what access having to a segment by opcodes like
VERR, VERW, LAR, LSL. They all set ZF if having access, clear if not.
First two simply verify R/W access, LAR gets bits defining access right
for a segment, LSL gives the segment limit value. These opcodes allow
checking what would cause access violation, instead getting the error.

Some instructions are allowed at CPL=0 only. They are:
Clear TaskÄSwitched Flag (CLTS), Halt Processor (HLT), loading some
system registers (GDTR,IDTR,LDTR,MSW,TR), any access to CRx,DRx,TRx.
Some other require CPL<=IOPL. They are: IN, INS, OUT, OUTS, CLI, STI.
Also, POPF behavior depends on CPL: if CPL>0, IOPL and VM aren't
changed by POPF, if CPL>IOPL, IF (interrupt enable) isn't changed.


Interrupts.

In every mode, there is an array containing information what action is
to be taken in case of interrupt. Its first entry corresponds to INT 0,
next to INT 1, and so on. It is called IDT(Interrupt Descriptor Table).
In RM, each entry in the IDT is simply far address of interrupt service
routine. Initially IDT is located at address 0 and has 100h entries
(400h bytes; some CPU-s have its limit 0FFFFh but the remainder isn't
accessible in RM); on pre-80286 CPUs the IDT address and size cannot be
changed, on 286+ can load and store them using LIDT and SIDT opcodes.

In PM the IDT has 8-byte entries which can be interrupt, trap or task
gates. Trap differs from interrupt by leaving interrupt flag same as
in interrupted code. Task gate causes calling another task. They all
have DPLs and interrupt instruction causes General Protection error
if CPL > interrupt or trap gate DPL. However, other interrupt sources
have "CPL 0" - they can access any gate needed.

Some conditions can cause an Exception. They are (for 80386): divide
error (0), debug exceptions (1), non-maskable interrupt (2), breakpoint
(3), overflow (4, on into opcode), bounds check (5, on bound opcode),
invalid opcode (6), coprocessor not available (7), double fault (8,E),
coprocessor segment overrun (9,P), invalid TSS (10,PE), segment not
present (11,PE), stack error (12,E), general protection error (13,E),
page fault (14,PE), coprocessor error (16); marked by P can occur in
PM and VM86 only, marked by E push error code on stack if they occur
in PM or VM86 (so stack is: error, IP, CS, flags; the error code is
usually either 0 or selector causing the exception (in case selector is
invalid or non-accessible), with flags on low order bits: bit 0 means
external source, bit 1 IDT selector, bit 2 LDT; for page fault it is
set of flags (bits 3-31 undefined): bit 0 set if page protection
violation, 1 if writing, 2 if user mode), most of them push IP of
opcode causing them, except 3,4,9 which push IP of next opcode.
Note: interrupt cannot be serviced at PL>CPL (unless via task switch),
attempt to do it causes General Protection error.

Interrupt processing in PM is more complicated when interrupt handler
has Privilege Level other than current code. It is handled similarly
CALL via gate: stack is switched, new SS:SP are taken from TSS, old
SS:SP are pushed on the new stack, then flags, CS, IP and eventually
error code (for some exceptions) are pushed.
In VM86 interrupt pushes GS,FS,DS,ES,SS,ESP,EFLAGS,CS,EIP (exception
also error code) onto PL 0 stack. There is VM bit in EFLAGS set to tell
interrupt occured in VM86. Note IDT must contain task gates and 80386
trap or interrupt gates pointing to a non-conforming code segment with
DPL=0 only - interrupt service must come through PL 0 or task switch.
The VM86 itself has CPL 3 and is allowed in 386 task only.


Descriptor Tables (PM only).

Global Descriptor Table(GDT) can contain descriptors of any type except
interrupt and trap gates. It is necessary for PM. First entry in GDT
isn't used - it corresponds to null selector which can be loaded into
segment register but causes exception if used for memory addressing.

Local Descriptor Table(LDT) can contain "normal" segment descriptors
(not e.g. TSS) and call or task gates only. Usually every task has its
own LDT (changed on task switch). The LDT must have descriptor in GDT.

Interrupt Descriptor Table(IDT) was discussed in "Interrupts" section.

"Normal" segment descriptors are referenced when a segment register is
loaded and they describe a memory area and give some access to it.
Bit 2 of selector used selects table: 0 means GDT, 1 means LDT.
Other descriptors can be Task State Segment(TSS), and gates. They can
be referenced "as a code segment", e.g. by far jump or call and they
cause transferring control to task or code segment referenced by them.
It is kind of indirect jump or call (they contain target selector).
TSS or gate pointing to TSS cause task switch. Gate can be used to
transfer control to more privileged code not accessible directly.
TSS can be also referenced by LTR (Load Task Register) opcode and it
is done once during PM initialization. LDT descriptor can be loaded
into LDTR(register) by LLDT opcode and usually it is done once.


Segment and System Descriptors.

The following segment types (in byte [descriptor+5]) are supported
(for all bit 7 means present in memory, bits 5-6 keep DPL which says
what is maximum CPL which can access the descriptor, the restriction is
for all descriptors, not segments only, except conforming segments):

10h+flags	- data: bit 1 - writable, bit 2 - expand down
18h+flags	- code: bit 1 - readable, bit 2 - conforming

for both, bit 0 is set by any access. The descriptor also contains
limit in word [0] (in 386 segments extended to bits 0-3 of byte [6])
and base in bytes [2..4] (in 386 segments extended to byte [7]).
Byte [6] keeps few additional flags: bit 7 - granularity (limit is in
4kB pages; e.g. limit 0 means 0..0FFFh accessible), bit 6 - 32-bit
addressing (applies to code and stack - use EIP, ESP, makes expand down
segment upper limit 4GB), bit 5 must be 0, bit 4 is for programmer.

01h+flags	- TSS: bit 1 - busy, bit 3 - 386 TSS
02h		- LDT
04h+flags	- call gate
05h		- task gate
06h+flags	- interrupt gate: bit 0 - trap, bit 3 - 386.

for all gates, word[2] keeps selector, word[0] and word[3] keep offset
of called code (ignored for task gate), byte[4] keeps word count (0-31)
for copying in case of inter-level call (call gate only, else ignored);
TSS and LDT have base and limit in same form as code and data segments
have, they can have bit 7 set in byte [6] to specify limit in pages.
Word [6] should be 0 for the descriptor to mean the same on 286/386.

LDT is similar GDT, except not all descriptor types are allowed.
TSS holds entire task state (all registers: general, segment, flags,
ip, ldtr); it also keeps link to caller TSS (valid if the task was
activated by INT or CALL) and stacks (SS and [E]SP) for PL 0,1,2
(they are used when more privileged code is invoked via gate from less
privileged). 386 TSS has also debug trap bit (if set, causes INT 1 on
task switch to the TSS), I/O bit map (saying which I/O addresses can
be accessed when CPL>IOPL without General Protection exception), and
CR3 value for the task (can remap memory on task switch).


Page tables:

both page directory and page table entries keep referenced address in
bits 31-12, have bits 11-9 reserved for programmer, must have bits 8,7,
4,3 set to 0; bit 5 is called A (accessed), it is set by CPU on access
to the entry, bit 6 is called D (dirty), it is set if referenced memory
is written; bit 0 is called P (present), all other are ignored if it is
not set; bit 2 allows user (CPL=3) access if set, bit 1 allows user to
write (together with bit 2 only), for CPL<3 read/write is allowed for
any setting of bits 1 and 2 (no protection against system this way).
Note page table entries used are usually cached by CPU: modifying them
in memory may cause no mapping change until the cache is reloaded. The
cache is flushed every time CR3 (which points to first page directory
entry) is loaded. Bits 0-11 of CR3 must be 0 (directory page-aligned).
Addressing through page tables: CR3+(Linear_Address SHR 20) AND 0FFCh
is address in Page Directory, the entry at the address contains Page
Table address; Page Table address + (Linear_Address SHR 10) AND 0FFCh
is address in Page Table and the entry at the address contains base
address of the page, combine it with bits 11-0 of Linear_Address and
the result is Physical Address. In case of any error, CR2 is set to the
Linear Address causing the error and error code explains what error.
Note: if Paging is enabled, CR3 must keep Physical Address of Page
Directory and all other addresses are Linear Addresses.


Switching to Protected Mode or back to Real Mode:

First: to get control in case of crash, need store in dword [0467h]
address where control is to be passed, and put 0Ah in CMOS register 0Fh
(by CLI; MOV AL,8Fh; OUT 70h,AL; (1us delay) MOV AL,0Ah; OUT 71h,AL;).
Also: normally, some circuitry in PC compatibles disables address line
A20; must enable it. If you use HIMEM, it can be enabled by a request
to HIMEM. If you also have DOS=HIGH, it is usually enabled, as it is
enabled by any DOS call. In other cases, you must send output port
value to keyboard controller to enable it before switching to PM.

Switch to PM: required is loading GDTR, then can enable protection by
setting CR0/MSW bit 0 (MOV EAX,CR0; OR AL,1; MOV CR0,EAX; or SMSW AX;
OR AL,1; LMSW AX; first on 386+, second on 286+); it is recommended
to load IDTR immediately before or after mode switch (same IDT can't be
valid in both modes); immediately after mode change should execute JMP
to flush prefetch queue which may be partially decoded (the decoding
may be mode dependent); need load CS and SS - they contain invalid
selectors and e.g. interrupt causes them to be put on stack and crash
on IRET; it is also recommended to load all segment registers (they can
be loaded with 0 to contain invalid selector and cause exception if any
of them is used to address memory) and LDTR; before first task switch
must load TR (selector of valid free TSS descriptor; the TSS will be
used to store state on task switch).

These is also a BIOS call which switches to PM and changes external
interrupt vector mapping (normally 1st controller has 08h..0Fh, 2nd
70h..77h, the 1st conflicts with some CPU exceptions; however it is
easy to distinguish external interrupt from an exception), it also
enables address line A20. See INT 15h, AH=89h description.

Returning to RM: it can be done by clearing bit 0 in CR0 but it needs
some preparation: must disable paging (go to code/stack which has
linear addresses same as physical, clear PG bit in CR0, clear CR3), go
to code segment with limit=64k and load all segment registers except
CS with valid descriptor of 64kB read/write expand-up byte-granular
present segment (attribute byte=93h, extended attribute=0) - otherwise
you can get RM with e.g. read-only or 32kB ES, which will soon cause
crash. After clearing the bit 0 of CR0 execute far jump to load CS and
flush prefetch queue and load segment registers for RM.

This is not available on 80286 which has no CR0 register (the Protect
Enable bit cannot be cleared by LMSW). The only way to get to RM again
is resetting the CPU: it can be done by the following code: CLI;
XOR CX,CX; wait_kbd_ctrlr_input_empty: IN AL,64h; TEST AL,2; LOOPNZ
wait_kbd_ctrlr_input_empty; MOV AL,0FEh; OUT 64h,AL; HLT; or by CPU
shutdown (resulting in case of exception while servicing double fault).

Note most programs running system in VM86 provide interface to switch
to PM and back to VM86, it is called VCPI (Virtual Control Program
Interface), can be tested for presence and invoked by INT 67h,AH=0DEh.
It requires 3 entries in GDT to be reserved for VCPI provider.

