Macros used in setting up descriptors

Some assembler macros are defined in sched.h and system.h to ease access and setting of descriptors. Each TSS entry and LDT entry takes 8 bytes.

Manipulating GDT system descriptors:

• _TSS(n),
  _LDT(n) These provide the index into the GDT for the n'th task.
• _LDT(n) is stored in the the ldt field of the tss_struct by fork.
• _set_tssldt_desc(n, addr, limit, type)
  ulong *n points to the GDT entry to set (see fork.c). The segment base (TSS or LDT) is set to 0xc0000000 + addr. Specific instances of the above are, where ltype refers to the byte containing P, DPL, S and type:

  

• load_TR(n),
  load_ldt(n) load descriptors for task number n into the task register and ldt register.
• ulong get_base (struct desc_struct ldt) gets the base from a descriptor.
• ulong get_limit (ulong segment) gets the limit (size) from a segment selector. Returns the size of the segment in bytes.
• set_base(struct desc_struct ldt, ulong base),
  set_limit(struct desc_struct ldt, ulong limit)
  Will set the base and limit for descriptors (4K granular segments). The limit here is actually the size in bytes of the segment.
• _set_seg_desc(gate_addr, type, dpl, base, limit)
  Default values 0x00408000 => D=1, P=1, G=0
  Present, operation size is 32 bit and max size is 1M.
  gate_addr must be a (ulong *)