TI486 Microprocessor - Al Kossow's Bitsavers

Register Set
Figure 2-9. Gate Descriptor
Call Gate descriptors are used to enter a procedure (subroutine) that executes
at the same or a more privileged level. A Call Gate descriptor primarily defines
the procedure entry point and the procedure's privilege level.
31 16 15 14 13 12 11 8 7 o
OFFSET 31-16 P DPL 0 TYPE 0 0 0 PARAMETERS +4
SELECTOR 15-0 OFFSET 15-0 +0
Table 2-7. Gate Descriptor Bit Definitions
BIT
POSITION
MEMORY
OFFSET
NAME
DESCRIPTION
31-16 +4 OFFSET Offset used during a call gate to calculate the branch target.
15-0 +0
31-16 +0 SELECTOR Segment selector used during a call gate to calculate the branch target.
15 +4 P Segment present
14-13 +4 DPL Descriptor privilege level
11-8 +4 TYPE Segment type:
0100=16-bit call gate
0101 =tack gate
0110=16-bit interrupt gate
0111 =16-bit trap gate
11 00=32-bit call gate
1110=32-bit interrupt gate
1111 =32-bit trap gate
4-0 +4 Parameters Number of 32-bit parameters to copy from the caller's stack to the called
procedure's stack.
2.3.2.3 Task Register
The Task Register (TR) holds a 16-bit selector for the current Task State
Segment (TSS) table as shown in Figure 2-10. The TR is loaded and stored
via the L TR and STR instructions, respectively. The TR can be accessed only
during protected mode and can be loaded only when the privilege level is 0
(most privileged).
Figure 2-10. Task Register
15 o
SELECTOR
When the TR is loaded, the TR selector field indexes a TSS descriptor that
must reside in the global descriptor table (GDT). The contents of the selected
descriptor are cached on-chip in the hidden portion of the TR.
2-24
Programming Interface