// This file contains definitions for the 
// x86 memory management unit (MMU).

// Eflags register
#define FL_CF           0x00000001      // Carry Flag
#define FL_PF           0x00000004      // Parity Flag
#define FL_AF           0x00000010      // Auxiliary carry Flag
#define FL_ZF           0x00000040      // Zero Flag
#define FL_SF           0x00000080      // Sign Flag
#define FL_TF           0x00000100      // Trap Flag
#define FL_IF           0x00000200      // Interrupt Enable
#define FL_DF           0x00000400      // Direction Flag
#define FL_OF           0x00000800      // Overflow Flag
#define FL_IOPL_MASK    0x00003000      // I/O Privilege Level bitmask
#define FL_IOPL_0       0x00000000      //   IOPL == 0
#define FL_IOPL_1       0x00001000      //   IOPL == 1
#define FL_IOPL_2       0x00002000      //   IOPL == 2
#define FL_IOPL_3       0x00003000      //   IOPL == 3
#define FL_NT           0x00004000      // Nested Task
#define FL_RF           0x00010000      // Resume Flag
#define FL_VM           0x00020000      // Virtual 8086 mode
#define FL_AC           0x00040000      // Alignment Check
#define FL_VIF          0x00080000      // Virtual Interrupt Flag
#define FL_VIP          0x00100000      // Virtual Interrupt Pending
#define FL_ID           0x00200000      // ID flag

// Segment Descriptor
struct segdesc {
  uint lim_15_0 : 16;  // Low bits of segment limit
  uint base_15_0 : 16; // Low bits of segment base address
  uint base_23_16 : 8; // Middle bits of segment base address
  uint type : 4;       // Segment type (see STS_ constants)
  uint s : 1;          // 0 = system, 1 = application
  uint dpl : 2;        // Descriptor Privilege Level
  uint p : 1;          // Present
  uint lim_19_16 : 4;  // High bits of segment limit
  uint avl : 1;        // Unused (available for software use)
  uint rsv1 : 1;       // Reserved
  uint db : 1;         // 0 = 16-bit segment, 1 = 32-bit segment
  uint g : 1;          // Granularity: limit scaled by 4K when set
  uint base_31_24 : 8; // High bits of segment base address
};

// Normal segment
#define SEG(type, base, lim, dpl) (struct segdesc)                      \
{ ((lim) >> 12) & 0xffff, (base) & 0xffff, ((base) >> 16) & 0xff,       \
    type, 1, dpl, 1, (uint) (lim) >> 28, 0, 0, 1, 1,                    \
    (uint) (base) >> 24 }

#define SEG16(type, base, lim, dpl) (struct segdesc)                    \
{ (lim) & 0xffff, (base) & 0xffff, ((base) >> 16) & 0xff,               \
    type, 1, dpl, 1, (uint) (lim) >> 16, 0, 0, 1, 0,                    \
    (uint) (base) >> 24 }

#define DPL_USER    0x3     // User DPL

// Application segment type bits
#define STA_X       0x8     // Executable segment
#define STA_E       0x4     // Expand down (non-executable segments)
#define STA_C       0x4     // Conforming code segment (executable only)
#define STA_W       0x2     // Writeable (non-executable segments)
#define STA_R       0x2     // Readable (executable segments)
#define STA_A       0x1     // Accessed

// System segment type bits
#define STS_T16A    0x1     // Available 16-bit TSS
#define STS_LDT     0x2     // Local Descriptor Table
#define STS_T16B    0x3     // Busy 16-bit TSS
#define STS_CG16    0x4     // 16-bit Call Gate
#define STS_TG      0x5     // Task Gate / Coum Transmitions
#define STS_IG16    0x6     // 16-bit Interrupt Gate
#define STS_TG16    0x7     // 16-bit Trap Gate
#define STS_T32A    0x9     // Available 32-bit TSS
#define STS_T32B    0xB     // Busy 32-bit TSS
#define STS_CG32    0xC     // 32-bit Call Gate
#define STS_IG32    0xE     // 32-bit Interrupt Gate
#define STS_TG32    0xF     // 32-bit Trap Gate

// PAGEBREAK: 40
// Task state segment format
struct taskstate {
  uint link;         // Old ts selector
  uint esp0;         // Stack pointers and segment selectors
  ushort ss0;        //   after an increase in privilege level
  ushort padding1;
  uint *esp1;
  ushort ss1;
  ushort padding2;
  uint *esp2;
  ushort ss2;
  ushort padding3;
  void *cr3;         // Page directory base
  uint *eip;         // Saved state from last task switch
  uint eflags;
  uint eax;          // More saved state (registers)
  uint ecx;
  uint edx;
  uint ebx;
  uint *esp;
  uint *ebp;
  uint esi;
  uint edi;
  ushort es;         // Even more saved state (segment selectors)
  ushort padding4;
  ushort cs;
  ushort padding5;
  ushort ss;
  ushort padding6;
  ushort ds;
  ushort padding7;
  ushort fs;
  ushort padding8;
  ushort gs;
  ushort padding9;
  ushort ldt;
  ushort padding10;
  ushort t;          // Trap on task switch
  ushort iomb;       // I/O map base address
};

// PAGEBREAK: 12
// Gate descriptors for interrupts and traps
struct gatedesc {
  uint off_15_0 : 16;   // low 16 bits of offset in segment
  uint cs : 16;         // code segment selector
  uint args : 5;        // # args, 0 for interrupt/trap gates
  uint rsv1 : 3;        // reserved(should be zero I guess)
  uint type : 4;        // type(STS_{TG,IG32,TG32})
  uint s : 1;           // must be 0 (system)
  uint dpl : 2;         // descriptor(meaning new) privilege level
  uint p : 1;           // Present
  uint off_31_16 : 16;  // high bits of offset in segment
};

// Set up a normal interrupt/trap gate descriptor.
// - istrap: 1 for a trap (= exception) gate, 0 for an interrupt gate.
//   interrupt gate clears FL_IF, trap gate leaves FL_IF alone
// - sel: Code segment selector for interrupt/trap handler
// - off: Offset in code segment for interrupt/trap handler
// - dpl: Descriptor Privilege Level -
//        the privilege level required for software to invoke
//        this interrupt/trap gate explicitly using an int instruction.
#define SETGATE(gate, istrap, sel, off, d)                \
{                                                         \
  (gate).off_15_0 = (uint) (off) & 0xffff;                \
  (gate).cs = (sel);                                      \
  (gate).args = 0;                                        \
  (gate).rsv1 = 0;                                        \
  (gate).type = (istrap) ? STS_TG32 : STS_IG32;           \
  (gate).s = 0;                                           \
  (gate).dpl = (d);                                       \
  (gate).p = 1;                                           \
  (gate).off_31_16 = (uint) (off) >> 16;                  \
}

