User:Johnburger/Demo/Exec/Alloc/LDT
This module provides four functions, to allocate LDTs and then to allocate Entries inside them.
Contents |
Allocating the memory for a new LDT is easy, but the code assumes that the LDT will need some tailoring done to it before it will become a true LDT. For that reason Exec.Alloc.LDT
returns a Data Segment, and expects Exec.Alloc.LDT.Enable
to be CALL
ed when finished so that it can convert it to a true LDT.
The last two functions use those found in Demo/Exec/Alloc/DT, which convert the passed-in parameters as necessary to make them fit inside the new Entry.
Exec.Alloc.LDT
This function allocates a new Data Segment with the passed-in size, and sets up the first couple of Entries in the nascent LDT for future Allocate purposes.
Exec.Alloc.LDT.Enable
This function converts the Data Segment into a true LDT.
Exec.Alloc.LDT.Sys
This Allocates a new System Descriptor in the LDT.
Exec.Alloc.LDT.Mem
This Allocates a new Memory Descriptor in the LDT.
Demo/Exec/Alloc/LDT.inc
; ; Exec/Alloc/LDT.inc ; ; This module provides functions to create LDTs and allocate entries in them. ;------------------------------------------------------------------------------- ; This function: ; 1) Allocates the requested RAM for an LDT; ; 2) Fills in the first entry with LDT.Alloc; ; 3) Fills in the second entry with an alias for this LDT (useful for debugging) ; 4) Returns a Data Descriptor for the LDT, ready for more customisation. ; To actually turn it into an LDT, call Exec.Alloc.LDT.Enable. ; ; Input: ECX = Size ; Output: AX = GDT Descriptor for (future) LDT, or zero on error ; ES = GDT Descriptor unless AX is zero Exec.Alloc.LDT: %push Alloc.LDT ; Let's not leave these %defines just lying around... %define %$Base EBP - 4 %define %$Size EBP - 8 ENTER 8, 0 MOV [%$Size], ECX CALL Exec.Alloc.RAM TEST EAX,EAX ; Any left? JZ .End ; Pity that! MOV [%$Base],EAX MOV EBX,EAX ; Base to allocate MOV DL,Type.Mem(Data, DPL0, RW) MOV DH,Gran.Mem(Byte, Small) CALL Exec.Alloc.GDT.Mem ; Get Descriptor to use TEST EAX,EAX ; Any left? JZ .End ; Pity that! **** Memory leak! **** MOV ES,AX ; Fresh Descriptor! MOV ECX,[%$Size] ; Bytes to zero XOR EDI,EDI ; Zero everything CALL Exec.Alloc.RAM.Zero ; Set up first entry as an LDT.Alloc MOV WORD [ES:LDT.Alloc.Limit],x86.Desc_size - 1 MOV BYTE [ES:LDT.Alloc.Type],x86.Desc.Sys.Type.LDT MOV [ES:LDT.Alloc.GDT],ES ; Set up second entry as an alias for this very LDT MOV EAX,[%$Base] ; Pass Base in MOV ECX,[%$Size] ; Size MOV DL,Type.Mem(Data, DPL0, RW) MOV DH,Gran.Mem(Byte, Small) CALL Exec.Alloc.LDT.Mem ; Allocate LDT entry MOV AX,ES ; Return value .End: LEAVE %pop RET ;------------------------------------------------------------------------------- ; This function converts a Segment previously allocated with Alloc.LDT (above) ; into a true LDT. ; ; Input: ES = Descriptor to modify ; Output: ES, EBX modified Exec.Alloc.LDT.Enable: ; Note that I could use GS: overrides to access the GDT, but ES points to the ; soon-to-be LDT - a Bad Thing. This fixes that! MOV EBX, ES ; Get Descriptor to modify MOV AX,GS ; Get GDT into ES MOV ES,AX MOV BYTE [ES:EBX+x86.Desc.Type],Type.Sys(LDT, DPL0, 286) RET ;------------------------------------------------------------------------------- ; This function allocates a new Desc.Sys in an LDT ; Input: ES = LDT to modify ; CX:EAX = Selector : Offset of handler ; DL = Descriptor Type ; Output: EAX = Allocated Descriptor, or zero on error ; EBX, ESI, EDI modified Exec.Alloc.LDT.Sys: PUSH EAX ; Save for later CALL Exec.Alloc.DT ; Allocate an Entry MOV EBX,EAX ; This is Descriptor to modify TEST EAX,EAX ; Any left? POP EAX ; It's later! JZ .Finish ; Uh oh! CALL Exec.Alloc.DT.Sys ; Call helper function .Finish: MOV EAX,EBX ; Return value RET ;------------------------------------------------------------------------------- ; This function allocates a new Desc.Mem in an LDT ; Input: ES = LDT to modify ; EAX = Base ; ECX = Size (Converted to Limit) ; DL = Descriptor Type ; DH = Descriptor Granularity ; Output: EAX = Allocated Descriptor, or zero on error ; EBX, ECX, DH, ESI, EDI modified Exec.Alloc.LDT.Mem: PUSH EAX ; Save for later CALL Exec.Alloc.DT ; Allocate an Entry MOV EBX,EAX ; This is Descriptor to modify TEST EAX,EAX ; Any left? POP EAX ; It's later! JZ .Finish ; Uh oh! CALL Exec.Alloc.DT.Mem ; Call helper function .Finish: MOV EAX,EBX ; Return value RET