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bernd.boec |
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; WMINICRT - minimal runtime for (Open) Watcom C to generate DOS .COM files
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;
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; The default behaviour of the runtime is:
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; - sets up a stack of 400H bytes
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; - releases additional memory beyond stack to DOS
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; - panics on initialization, if not enough memory for stack size
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; - panics if running out of stack during execution
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;
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; WASM definitions:
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; STACKSIZE=<value> define stack size other than 400h
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; NOSTACKCHECK do not assemble __STK function
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; STACKSTAT remember the minimum SP, maintained by __STK__
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; and exported via _stack_low_.
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;
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; To build an executable without build- and linker scripts run:
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; wasm startup.asm
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; wcc -0 -bt=dos -ms -os -zl your_c_file.c
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; wlink system dos com file startup,your_c_file
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; To change the stack size, add -DSTACKSIZE=<value> to the wasm call
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;
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; To compile without stack checking:
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; a) compile this startup file with wasm -DNOSTACKCHECK startup.asm
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; b) call the C compiler with -s flag
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;
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; To build a debug version of your program:
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; wasm -d1 startup.asm
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; wcc -bt=dos -ms -d2 -zl your_c_file.c
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; wlink system dos com debug all option map file startup,your_c_file
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;
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; TODO:
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; - display stack statistics on program termination if STACKSTAT is enabled
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; - proper Makefiles
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; - many more (optional) things while keeping it small :-)
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.8086
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IFNDEF STACKSIZE
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STACKSIZE = 400h
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ENDIF
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DGROUP group _TEXT,_DATA,CONST,CONST2,_BSS,_STACK
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extrn "C",main : near
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public _cstart_, _small_code_
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_TEXT segment word public 'CODE'
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org 100h
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_small_code_ label near
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_cstart_ proc
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; DOS puts the COM program in the largest memory block it can find
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; and sets SP to the end of this block. On top of that, it reserves
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; the entire memory (not only the process' block) to the program, which
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; makes it impossible to allocate memory or run child processes.
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; for this reasons it is beneficial to resize the memory block we occupy
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; into a more reasonable value
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@verify_stack_size:
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cmp sp,offset DGROUP:_stack_end_
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ja @resize_mem
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mov dx,offset @memerr
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jmp _panic_
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@memerr db 'MEMERR$'
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; step 2: resize our memory block to sp bytes (ie. sp/16 paragraphs)
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@resize_mem:
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mov sp,offset DGROUP:_stack_end_
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IFDEF STACKSTAT
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mov [_stack_low_],sp
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ENDIF STACKSTAT
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mov ah,4ah
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mov bx,sp
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add bx,0fh
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shr bx,1
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shr bx,1
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shr bx,1
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shr bx,1
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int 21h
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; clear _BSS to be ANSI C conformant
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mov di,offset DGROUP:_BSS
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mov cx,offset DGROUP:_STACK
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sub cx,di
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shr cx,1
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xor ax,ax
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cld
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rep stosw
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call main
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mov ah,4ch
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int 21h
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_cstart_ endp
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_panic_ proc
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; output error message in DX, then terminate with FF
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mov ah,9
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int 21h
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mov ax,4cffh ; terminate if not enough stack space
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int 21h
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_panic_ endp
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IFNDEF NOSTACKCHECK
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public __STK
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__STK proc
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; ensures that we have enough stack space left. the needed bytes are
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; given in AX. panics if stack low.
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sub ax,sp ; subtract needed bytes from SP
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neg ax ; SP-AX = -(AX-SP)
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cmp ax,offset DGROUP:_STACK - 2 ; -2 is to compensate for __STK ret addr
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jae @l1 ; enough stack => return, else panic
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int 3 ; trap into debugger
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mov dx,offset @stkerr
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add sp,200h ; make sure we have enough stack to call DOS
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jmp _panic_
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@stkerr db 'STKERR$'
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@l1:
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IFDEF STACKSTAT ; update lowest stack pointer if statistics enabled
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cmp [_stack_low_],ax
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jbe @r
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mov [_stack_low_],ax
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ENDIF STACKSTAT
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@r:
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ret
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__STK endp
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ENDIF
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_DATA segment word public 'DATA'
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_DATA ends
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CONST segment word public 'DATA'
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CONST ends
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CONST2 segment word public 'DATA'
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CONST2 ends
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_BSS segment word public 'BSS'
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IFDEF STACKSTAT
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public _stack_low_
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_stack_low_: dw 1 dup(?)
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ENDIF STACKSTAT
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_BSS ends
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; stack definition, available memory is checked at runtime
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; defined as segment so that linker may detect .COM size overflow
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_STACK segment para public 'TINY_STACK'
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public _stack_end_
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db STACKSIZE dup(?)
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_stack_end_:
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_STACK ends
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_TEXT ends
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end _cstart_
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