Q33626: Assembly Routine Using STD Works in QB 3.00, Not 4.00
Article: Q33626 Product(s): See article Version(s): 3.00 4.00 4.00B 4.50 Operating System(s): MS-DOS Keyword(s): ENDUSER | B_BasicCom | mspl13_basic Last Modified: 6-DEC-1989 An assembly-language subprogram that uses the STD instruction (sets the direction flag to indicate that strings will process down from high addresses to low addresses) may not print the string correctly in QuickBASIC Version 4.00 unless the flag is first cleared before returning, using the CLD instruction. Although this is the recommended way to program the assembly language subprogram, QuickBASIC Version 3.00 prints the string correctly even if the flag was not cleared. The following is a code example: 'This BASIC program calls the MASM subprogram RTrim, with a string variable 'and an integer as variables. Rtrim looks at the given string and returns 'the string's length, minus any trailing blank characters. DEFINT A-Z length = 0 INPUT "String: ", x$ 'Take a string, any string.... x$ = x$ + STRING$(2, 32) 'Add two trailing blank characters. PRINT x$; LEN(x$) 'Print the string and its length. CALL RTrim(x$, length) x$ = LEFT$(x$, length) 'Chop off any trailing blanks. PRINT x$; LEN(x$) 'Print the new string and its new length. END .MODEL MEDIUM .CODE PUBLIC RTrim RTrim PROC push bp mov bp,sp push es push ds ;Be sure ES and DS match. pop es mov bx,[bp+8] ;Get the string's descriptor. mov di,[bx+2] ;DI points to the string. mov cx,[bx] ;Get the string's length in CX. jcxz exit ;Length = 0? If it is, then exit. add di,cx ;DI points past the string. dec di ;DI now points to the last character in ;the string. std ;Direction: High to Low. mov al,20h ;Blank to look for. repe scasb ;Repeat until a character other than ;a blank is found. mov bx,[bp+6] ;Now get the word to store the length. jnz go_on ;If found then go ahead, mov word ptr [bx],0 ;else store 0 in the word pushed as 2nd jmp short exit ;parameter, then exit. go_on: inc cx mov [bx],cx ;Store it in Arg1. exit: cld ;****Need this to run with QuickBASIC 4.00. pop es ;Restore the initial values for ES and pop bp ;BP as required. ret 4 ;Pop off the two parameters before RTrim ENDP ;returning to caller. END The following are the results without the CLD instruction in QuickBASIC Version 4.00: String: abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz 28 a8 zyxwvutsrqponmlkjif 26 The following are the results in QuickBASIC Version 3.00, or with the CLD instruction in QuickBASIC Version 4.00: String: abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz 28 abcdefghijklmnopqrstuvwxyz 26
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