80C286 datasheet(22/65 Pages) RENESAS

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80C286

FN2947 Rev.3.00

Page 22 of 65

January 28, 2008

Control Transfer

Four types of control transfer can occur when a selector is

loaded into CS by a control transfer operation (see Table 13).

Each transfer type can only occur if the operation which loaded

the selector references the correct descriptor type. Any viola-

tion of these descriptor usage rules (e.g. JMP through a call

gate or RET to a Task State Segment) will cause exception 13.

The ability to reference a descriptor for control transfer is also

subject to rules of privilege. A CALL or JUMP instruction may

only reference a code segment descriptor with DPL equal to

the task CPL or a conforming segment with DPL of equal or

greater privilege than CPL. The RPL of the selector used to

reference the code descriptor must have as much privilege as

CPL.

RET and IRET instructions may only reference code segment

descriptors with descriptor privilege equal to or less privileged

than the task CPL. The selector loaded into CS is the return

address from the stack. After the return, the selector RPL is the

task's new CPL. If CPL changes, the old stack pointer is

popped after the return address.

When a JMP or CALL references a Task State Segment

descriptor, the descriptor DPL must be the same or less privi-

leged than the task's CPL. Reference to a valid Task State

Segment descriptor causes a task switch (see Task Switch

Operation). Reference to a Task State Segment descriptor at a

more privileged level than the task's CPL generates exception

13.

When an instruction or interrupt references a gate descriptor,

the gate DPL must have the same or less privilege than the

task CPL. If DPL is at a more privileged level than CPL, excep-

tion 13 occurs. If the destination selector contained in the gate

references a code segment descriptor, the code segment

descriptor DPL must be the same or more privileged than the

task CPL. If not, Exception 13 is issued. After the control trans-

fer, the code segment descriptors DPL is the task's new CPL. If

the destination selector in the gate references a task state seg-

ment, a task switch is automatically performed (see Task

Switch Operation).

The privilege rules on control transfer require:

• JMP or CALL direct to a code segment (code segment

descriptor) can only be a conforming segment with DPL of

equal or greater privilege than CPL or a non-conforming seg-

ment at the same privilege level.

• Interrupts within the task, or calls that may change privilege

levels, can only transfer control through a gate at the same or

a less privileged level than CPL to a code segment at the

same or more privileged level than CPL.

• Return instructions that don't switch tasks can only return con-

trol to a code segment at the same or less privileged level.

• Task switch can be performed by a call, jump or interrupt

which references either a task gate or task state segment at

the same or less privileged level.

Privilege Level Changes

Any control transfer that changes CPL within the task, causes

a change of stacks as part of the operation. Initial values of

SS:SP for privilege levels 0, 1, and 2 are kept in the task state

segment (refer to Task Switch Operation). During a JMP or

CALL control transfer, the new stack pointer is loaded into the

SS and SP registers and the previous stack pointer is pushed

onto the new stack.

When returning to the original privilege level, its stack is

restored as part of the RET or IRET instruction operation. For

subroutine calls that pass parameters on the stack and cross

privilege levels, a fixed number of words, as specified in the

gate, are copied from the previous stack to the current stack.

The inter-segment RET instruction with a stack adjustment

value will correctly restore the previous stack pointer upon

return.

Protection

The 80C286 includes mechanisms to protect critical instruc-

tions that effect the CPU execution state (e.g. HLT) and code

or data segments from improper usage. These protection

mechanisms are grouped into three forms:

• Restricted usage of segments (e.g. no write allowed to read-

only data segments). The only segments available for use are

defined by descriptors in the Local Descriptor Table (LDT)

and Global Descriptor Table (GDT).

• Restricted access to segments via the rules of privilege and

descriptor usage.

• Privileged instructions or operations that may only be exe-

cuted at certain privilege levels as determined by the CPL and

I/O Privilege Level (lOPL). The lOPL is defined by bits 14 and

13 of the flag word.

These checks are performed for all instructions and can be

split into three categories: segment load checks (Table 14),

operand reference checks (Table 15), and privileged instruc-

tion checks (Table 16). Any violation of the rules shown will

result in an exception. A not-present exception related to the

stack segment causes exception 12.

TABLE 14. SEGMENT REGISTER LOAD CHECKS

ERROR DESCRIPTION

EXCEPTION

NUMBER

Descriptor table limit exceeded

Segment descriptor not-present

11 or 12

Privilege rules violated

Invalid descriptor/segment type segment register

load:

- Read only data segment load to SS

- Special control descriptor load to DS, ES, SS

- Execute only Segment load to DS, ES, SS

- Data segment load to CS

- Read/Execute code segment load SS

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