User:Johnburger/Demo/x86/Desc
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This is both the most involved x86 definition file and the most useful. Since I decided to define as many system tables as possible at assembly time rather than run time, defining Descriptors became a frequent occurrence. And it didn't take long for NASM's structure instantiation mechanism (ISTRUC - *shudder*) to muddy the code enough to warrant a %macro to help.
I'm actually quite proud of the result - take a look at the Interrupt Descriptor Table definition for an example of how clean the resultant definition really is.
Demo/x86/Desc.inc
;
; x86/Desc.inc
;
; Descriptor Tables hold Descriptor Table Entries, or Descriptors for short.
; To select a Descriptor, you load a Segment register with a Selector (see Seg.inc).
; The following is the generic structure of a Descriptor (not very useful!)
STRUC x86.Desc
RESD 1
RESB 1
.Type RESB 1 ; All Descriptors have a Type field here
RESW 1
ENDSTRUC ; All Descriptors have the same Desc_size
; The following is the structure of a System Descriptor
STRUC x86.Desc.Sys
.OffsetLo RESW 1
.Selector RESW 1
.Args RESB 1
.Type RESB 1
.OffsetHi RESW 1
ENDSTRUC
; The following is the structure of a Memory (Code/Data) Descriptor
STRUC x86.Desc.Mem
.LimitLo RESW 1
.BaseLo RESW 1
.BaseMid RESB 1
.Type RESB 1
.Gran RESB 1
.BaseHi RESB 1
ENDSTRUC
; The .Type field in both .Sys and .Mem structures above have the same format.
; These are the bitfields of the .Type field
x86.Desc.Type.Present EQU 1000_0000b
; These are the different Descriptor Privilege Levels
x86.Desc.Type.DPL3 EQU 0110_0000b
x86.Desc.Type.DPL2 EQU 0100_0000b
x86.Desc.Type.DPL1 EQU 0010_0000b
x86.Desc.Type.DPL0 EQU 0000_0000b
x86.Desc.Type.Sys EQU 0000_0000b
x86.Desc.Type.Mem EQU 0001_0000b
x86.Desc.Type.Type EQU 0000_1111b
;-------------------------------------------------------------------------------
; These are the .Type.Type field values and modifiers for System Descriptors
x86.Desc.Sys.Type.TSS EQU 0000_0001b ; Task State Segment
x86.Desc.Sys.Type.LDT EQU 0000_0010b ; Local Descriptor Table
x86.Desc.Sys.Type.Busy EQU 0000_0011b ; Busy TSS
x86.Desc.Sys.Type.Gate EQU 0000_0111b ; One of a number of Gates
x86.Desc.Sys.Type.Call EQU 0000_0100b ; Call Gate (switches Priv)
x86.Desc.Sys.Type.Task EQU 0000_0101b ; Task Gate (switches TSS)
x86.Desc.Sys.Type.Int EQU 0000_0110b ; Int Gate (disables interrupts)
x86.Desc.Sys.Type.Trap EQU 0000_0111b ; Trap Gate
x86.Desc.Sys.Type.386 EQU 0000_1000b ; '386 variant of above
x86.Desc.Sys.Type.286 EQU 0000_0000b ; '286 is the original ones
; The .386 modifier is used for 32-bit variants of the original '286 16-bit ones.
; Note it cannot be used for either .LDT or .Task
;-------------------------------------------------------------------------------
; These are the .Type.Type field values and modifiers for Memory Descriptors
x86.Desc.Mem.Type.Code EQU 0000_1000b
x86.Desc.Mem.Type.Data EQU 0000_0000b
x86.Desc.Mem.Type.Conform EQU 0000_0100b ; Code can be executed at different PLs
x86.Desc.Mem.Type.Expand EQU 0000_0100b ; Base is TOP, Limit is size BELOW that
x86.Desc.Mem.Type.Stack EQU 0000_0100b ; (Best used for Stacks)
x86.Desc.Mem.Type.RW EQU 0000_0010b ; Readable for Code, Writable for Data
x86.Desc.Mem.Type.NoRW EQU 0000_0000b ; No Read for Code, no Write for Data
x86.Desc.Mem.Type.Accessed EQU 0000_0001b ; Set when Descriptor loaded
; These are the bitfields of the .Gran field in Memory Descriptors
x86.Desc.Mem.Gran.LimitHi EQU 0000_1111b
x86.Desc.Mem.Gran.Gran EQU 1000_0000b ; Use Page-granular Limit
x86.Desc.Mem.Gran.Byte EQU 0000_0000b ; Use byte-granular Limit
x86.Desc.Mem.Gran.Big EQU 0100_0000b ; Use EIP and ESP for Code and Stacks
x86.Desc.Mem.Gran.Small EQU 0000_0000b ; Use IP and SP for Code and Stacks
x86.Desc.Mem.Gran.Def32 EQU 0100_0000b ; Default to 32-bit instructions
x86.Desc.Mem.Gran.Def16 EQU 0000_0000b ; Default to 16-bit instructions
x86.Desc.Mem.Gran.Avail EQU 0001_0000b ; Available for system use
;-------------------------------------------------------------------------------
; Helper macros for each of the .Types defined above. There are a number of
; fields that need to be set, making for long source lines, and it's easy to
; forget one! Each of these assume only one thing: they're .Present
; Parameters are: TSS or LDT or Call or Task or Int or Trap, DPL 0-3, 286 or 386
%define Type.Sys(Type, DPL, x86) (\
x86.Desc.Type.Present | x86.Desc.Type.Sys | \
x86.Desc.Type.%+ DPL | x86.Desc.Sys.Type.%+ Type | x86.Desc.Sys.Type.%+ x86 \
)
; Parameters are: Code or Data or Stack, DPL 0-3, and RW or NoRW
%define Type.Mem(Type, DPL, RW) (\
x86.Desc.Type.Present | x86.Desc.Type.Mem | \
x86.Desc.Type.%+ DPL | x86.Desc.Mem.Type.%+ Type | x86.Desc.Mem.Type.%+ RW \
)
; Parameters are: Gran or Byte, and Big or Small
%define Gran.Mem(Gran, Size) (\
x86.Desc.Mem.Gran.%+ Gran | x86.Desc.Mem.Gran.%+ Size \
)
; A helper macro, making it easy to allocate a System Descriptor in a Table.
; Unfortunately this macro requires that the Offset is less than 64K.
; Tne parameters are: Selector, Offset, Type
%macro Desc.Sys 3
%00 ISTRUC x86.Desc.Sys
AT x86.Desc.Sys.OffsetLo, DW %2
AT x86.Desc.Sys.Selector, DW %1
AT x86.Desc.Sys.Args, DB 0
AT x86.Desc.Sys.Type, DB %3
AT x86.Desc.Sys.OffsetHi, DW 0
IEND
%endmacro
; A helper macro, making it easy to allocate a Memory Descriptor in a Table.
; The parameters are: Base, Limit, Type, and Gran
%macro Desc.Mem 4
%00 ISTRUC x86.Desc.Mem
AT x86.Desc.Mem.LimitLo, DW %2
AT x86.Desc.Mem.BaseLo, DW (%1 >> 0) & 0FFFFh
AT x86.Desc.Mem.BaseMid, DB (%1 >> 16) & 0FFh
AT x86.Desc.Mem.Type, DB %3
AT x86.Desc.Mem.Gran, DB %4
AT x86.Desc.Mem.BaseHi, DB (%1 >> 24) & 0FFh
IEND
%endmacro