User:Johnburger/Demo/x86/Desc
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< User:Johnburger | Demo
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
