162 lines
6.2 KiB
NASM
162 lines
6.2 KiB
NASM
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;
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cr equ 0dh ;ASCII carriage return
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lf equ 0ah ;ASCII line feed
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;
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cseg segment para public 'CODE'
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;
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assume cs:cseg,ds:data,ss:stack
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;
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demo proc far
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;at entry DS & ES = PSP
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push ds ;Save address for final
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xor ax,ax ;FAR RET to PC-DOS on stack
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push ax
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;save copy of SS:SP for use
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;after return from overlay
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mov cs:STK_SEG,ss
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mov cs:STK_PTR,sp
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;
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;Reserve 1000H bytes for
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;this loader and release
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;the rest of memory for
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;use by the overlayed program.
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mov bx,100h ;ES=segment of PSP of loader
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mov ah,4ah ;BX=paragraphs to reserve
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int 21h
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;make the messages in data
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;segment addressable
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mov ax,seg DATA
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mov ds,ax
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mov es,ax
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;jump if memory
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;de-allocation failed
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jc ALLOC_ERR
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;print memory successfully
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;released
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mov dx,offset MSG2
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mov ah,9
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int 21h
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;
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;now load and execute
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;the overlaid program.
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mov dx,offset PGM_NAME
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mov bx,offset PAR_BLK
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mov al,0
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mov ah,4bh
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int 21h
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;restore stack pointers
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;to state before EXEC call
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mov ss,cs:STK_SEG
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mov sp,cs:STK_PTR
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;Make data segment
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;addressable again
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mov ax,seg DATA
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mov ds,ax
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;print message that loader
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;successfully regained control
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mov dx,offset MSG3
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mov ah,9
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int 21h
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;now exit to PC-DOS
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ret
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alloc_err: ;come here if memory
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;cannot be released
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mov dx,offset MSG1
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mov ah,9
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int 21h ;print error message and
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ret ;exit to PC-DOS
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;
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demo endp
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;
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;these two variables must
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;reside in Code Segment so
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;that they are addressable
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;after return from overlay.
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stk_seg dw 0 ;original SS contents
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stk_ptr dw 0 ;original SP contents
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;
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cseg ends
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;declare a stack area
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;for use by this loader
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stack segment para stack 'STACK'
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;allow 64 bytes in this case
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db 64 dup (?)
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stack ends
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;declare data segment to
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;contain variables and tables
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data segment para public 'DATA'
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;
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msg1 db cr,lf
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db 'Unable to release memory.'
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db cr,lf,'$'
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msg2 db cr,lf
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db 'Memory above loader released.'
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db cr,lf,'Now loading CHKDSK.COM.'
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db cr,lf,'$'
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msg3 db cr,lf
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db 'Loader regained control from CHKDSK,'
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db cr,lf
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db 'now making final exit to PC-DOS.'
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db cr,lf,'$'
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;
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;drive, path, and name of program
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;to be loaded and executed.
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pgm_name db '\CHKDSK.COM',0
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;
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par_blk dw ENVIR ;segment address of
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;environment descriptor
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;
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;full address of command line
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;to be passed at offset 80H
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dw offset CMD_LINE ;in overlaid
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dw seg CMD_LINE ;program's PSP
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;
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;full address of default
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;File Control Block to be
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;passed at offset 5CH in
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dw offset FCB1 ;overlaid
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dw seg FCB1 ;program's PSP
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;
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;full address of default
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;File Control Block to be
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;passed at offset 6CH in
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dw offset FCB2 ;overlaid
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dw seg FCB2 ;program's PSP
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;
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;actual command line tail
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;to be passed to overlay
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cmd_line db 4,' *.*',cr,0
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;
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;first default FCB to
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fcb1 db 0 ;be passed to overlay
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db 11 dup ('?')
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db 25 dup (0)
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;second default FCB to
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fcb2 db 0 ;be passed to overlay
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db 11 dup (' ')
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db 25 dup (0)
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;
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data ends
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;declare separate data
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;segment to contain
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;environment descriptor
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envir segment para 'ENVIR'
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;
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;Search path used by PC-DOS
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;to look for commands or
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;batch files not found in
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db 'PATH=',0 ;the current directory
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;
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;Search path used by PC-DOS
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;to locate COMMAND.COM
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db 'COMSPEC=A:\COMMAND.COM',0
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db 0 ;extra 0 byte designates
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;end of environment
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envir ends
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end demo
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