This preview shows page 1. Sign up to view the full content.
Unformatted text preview: offset of a pointer. The segment limit is a 20-bit value contained in bytes 0 and 1 and in the first 4 bits of byte 6 of the segment descriptor. If the descriptor has a byte granular segment limit (the granularity flag is set to 0), the destination operand is loaded with a byte granular value (byte limit). If the descriptor has a page granular segment limit (the granularity flag is set to 1), the LSL instruction will translate the page granular limit (page limit) into a byte limit before loading it into the destination operand. The translation is performed by shifting the 20-bit "raw" limit left 12 bits and filling the loworder 12 bits with 1s. When the operand size is 32 bits, the 32-bit byte limit is stored in the destination operand. When the operand size is 16 bits, a valid 32-bit limit is computed; however, the upper 16 bits are truncated and only the low-order 16 bits are loaded into the destination operand. This instruction performs the following checks before it loads the segment limit into the destination register: Checks that the segment selector is not NULL. Checks that the segment selector points to a descriptor that is within the limits of the GDT or LDT being accessed Checks that the descriptor type is valid for this instruction. All code and data segment descriptors are valid for (can be accessed with) the LSL instruction. The valid special segment and gate descriptor types are given in the following table. 3-550 Vol. 2 INSTRUCTION SET REFERENCE, A-M If the segment is not a conforming code segment, the instruction checks that the specified segment descriptor is visible at the CPL (that is, if the CPL and the RPL of the segment selector are less than or equal to the DPL of the segment selector). If the segment descriptor cannot be accessed or is an invalid type for the instruction, the ZF flag is cleared and no value is loaded in the destination operand. Table 3-60. Segment and Gate Descriptor Types
Type 0 1 2 3 4 5 6 7 8 9 A B C D E F Reserved Available 16-bit TSS LDT Busy 16-bit TSS 16-bit call gate 16-bit/32-bit task gate 16-bit interrupt gate 16-bit trap gate Reserved Available 32-bit TSS Reserved Busy 32-bit TSS 32-bit call gate Reserved 32-bit interrupt gate 32-bit trap gate Protected Mode Name Valid No Yes Yes Yes No No No No No Yes No Yes No No No No IA-32e Mode Name Upper 8 byte of a 16Byte descriptor Reserved LDT Reserved Reserved Reserved Reserved Reserved Reserved 64-bit TSS Reserved Busy 64-bit TSS 64-bit call gate Reserved 64-bit interrupt gate 64-bit trap gate Valid Yes No Yes No No No No No No Yes No Yes No No No No Vol. 2 3-551 INSTRUCTION SET REFERENCE, A-M Operation
IF SRC(Offset) > descriptor table limit THEN ZF 0; FI; Read segment descriptor; IF SegmentDescriptor(Type) conforming code segment and (CPL > DPL) OR (RPL > DPL) or Segment type is not valid for instruction THEN ZF 0; ELSE temp SegmentLimit([SRC]); IF (G 1) THEN temp ShiftLeft(12, temp) OR 00000FFFH; ELSE IF OperandSize = 32 THEN DEST temp; FI; EL...
View Full Document
- Winter '11