User:Johnburger/Demo/Pad/Sizes

Each of the Segments in their code may need a separate .Base, .Size and .Limit defined for them, so that various fields in various system structures can be filled in. Also, we need to know sizes so that the Loader can work out how much it has to load. This is accumulated inside the Demo.Size variable.

This module simply re-opens each of the Segments defined so far in the system, and defines the necessary symbols. Note that the GDT is handled specially: it will grow through the system's lifetime, so we need to know its current .Limit, but not get confused with its final full .Size.

Demo/Pad/Sizes.inc

;
; Pad/Sizes.inc
;
 
; Calculate final Real Segment size
                SEGMENT         Real ALIGN=16
 
Real.Base       EQU             BIOS.Entry
 
                ALIGN           16
 
Real.Size       EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + Real.Size
;-------------------------------------------------------------------------------
; Calculate final IDT Segment size
                SEGMENT         IDT  ALIGN=16
 
IDT.Start       EQU             Code.Base - IDT.Size
 
IDT.Limit       EQU             $ - $$ - 1
 
                ALIGN           16, DB 0
 
IDT.Size        EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + IDT.Size
;-------------------------------------------------------------------------------
; Calculate final Ints Segment size
                SEGMENT         Ints  ALIGN=16
 
Ints.Base       EQU             IDT.Start + IDT.Size ; Equivalent to Code.Base
 
Ints.Limit      EQU             $ - $$ - 1
 
                ALIGN           16
 
Ints.Size       EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + Ints.Size
;-------------------------------------------------------------------------------
; Calculate final User.LDT Segment size
                SEGMENT         User.LDT  ALIGN=16  NOBITS
 
User.LDT.Limit  EQU             $ - $$ - 1
 
                ALIGNB          16
 
User.LDT.Size   EQU             $ - $$
;-------------------------------------------------------------------------------
; Calculate final User.Data Segment size
                SEGMENT         User.Data  ALIGN=16  NOBITS
 
User.Data.Limit EQU             $ - $$ - 1
 
                ALIGNB          16
 
User.Data.Size  EQU             $ - $$
;-------------------------------------------------------------------------------
; Calculate final User.Code Segment size
                SEGMENT         User  ALIGN=16
 
User.Base       EQU             Ints.Base + Ints.Size
 
User.Limit      EQU             $ - $$ - 1
 
                ALIGN           16
 
User.Size       EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + User.Size
;-------------------------------------------------------------------------------
; Calculate final Exec Segment size
                SEGMENT         Exec  ALIGN=16
 
Exec.Base       EQU             User.Base + User.Size
 
Exec.Limit      EQU             $ - $$ - 1
 
                ALIGN           16
 
Exec.Size       EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + Exec.Size
;-------------------------------------------------------------------------------
; Calculate final Exec.LDT Segment size
                SEGMENT         Exec.LDT  ALIGN=16
 
Exec.LDT.Base   EQU             Exec.Base + Exec.Size
 
Exec.LDT.Limit  EQU             $ - $$ - 1
 
                ALIGN           16, DB 0
 
Exec.LDT.Size   EQU             $ - $$
 
%assign         Demo.Size       Demo.Size + Exec.LDT.Size
;-------------------------------------------------------------------------------
; Calculate final GDT Segment size
                SEGMENT         GDT  ALIGN=16
 
GDT.Base        EQU             Exec.LDT.Base + Exec.LDT.Size
 
GDT.Limit       EQU             $ - $$ - 1
 
; Note that GDT grows from here - no alignment necessary
 
%assign         Demo.Size       Demo.Size + GDT.Limit + 1