Question

我需要使用Int 21h / function 23h获取文件大小。我不知道FCB是如何工作的，也不知道如何使用它。此documentation中给出的函数23h的描述是：

功能23h（35）使用FCB获取文件大小

Returns the number of records in a file.

  On entry:      AH         23h
                 DS:DX      Pointer to an unopened FCB

  Returns:       AL         00h      If matching file found
                            FFh      If no matching file found

我想如何使用它？

Answer 1

您链接的同一页面底部有FCB的链接。如果您遵循它，您将看到您需要为FCB初始化一块内存，并使用驱动器号（03代表C :)，8.3格式的文件名以及其他几个字段填充它。将ds：dx指向该内存块，将ah设置为23h，int 21。如果al为0，则表示成功，您可以从ds：dx + 10h获取文件长度。

请参阅偏移量为0x10的b2 6a 00 00，它与指定文件的大小相匹配：

jcomeau@aspire:/tmp$ ls -l /usr/src/dosdebug/DEBUG.TXT
-rw-r--r-- 1 jcomeau jcomeau 27314 Feb 15  2014 /usr/src/dosdebug/DEBUG.TXT
jcomeau@aspire:/tmp$ printf '%x\n' 27314
6ab2

我必须修补dosbox中的一个错误才能使其正常工作，因此更新速度很慢。

diff -up ./include/dos_inc.h.orig ./include/dos_inc.h
--- ./include/dos_inc.h.orig    2015-12-27 12:43:12.000000000 -0800
+++ ./include/dos_inc.h 2015-12-27 11:53:19.000000000 -0800
@@ -170,7 +170,7 @@ void DOS_BuildUMBChain(bool umb_active,b
 bool DOS_LinkUMBsToMemChain(Bit16u linkstate);

 /* FCB stuff */
-bool DOS_FCBOpen(Bit16u seg,Bit16u offset);
+Bit16u DOS_FCBOpen(Bit16u seg,Bit16u offset);
 bool DOS_FCBCreate(Bit16u seg,Bit16u offset);
 bool DOS_FCBClose(Bit16u seg,Bit16u offset);
 bool DOS_FCBFindFirst(Bit16u seg,Bit16u offset);
diff -up ./src/dos/dos_classes.cpp.orig ./src/dos/dos_classes.cpp
diff -up ./src/dos/dos_files.cpp.orig ./src/dos/dos_files.cpp
--- ./src/dos/dos_files.cpp.orig    2015-12-27 10:08:26.000000000 -0800
+++ ./src/dos/dos_files.cpp 2015-12-27 12:22:17.000000000 -0800
@@ -964,7 +964,7 @@ bool DOS_FCBCreate(Bit16u seg,Bit16u off
    return true;
 }

-bool DOS_FCBOpen(Bit16u seg,Bit16u offset) { 
+Bit16u DOS_FCBOpen(Bit16u seg,Bit16u offset) { 
    DOS_FCB fcb(seg,offset);
    char shortname[DOS_FCBNAME];Bit16u handle;
    fcb.GetName(shortname);
@@ -972,7 +972,7 @@ bool DOS_FCBOpen(Bit16u seg,Bit16u offse
    /* First check if the name is correct */
    Bit8u drive;
    char fullname[DOS_PATHLENGTH];
-   if (!DOS_MakeName(shortname,fullname,&drive)) return false;
+   if (!DOS_MakeName(shortname,fullname,&drive)) return (Bit16u)false;

    /* Check, if file is already opened */
    for (Bit8u i=0;i<DOS_FILES;i++) {
@@ -982,16 +982,16 @@ bool DOS_FCBOpen(Bit16u seg,Bit16u offse
            if (handle==0xFF) {
                // This shouldnt happen
                LOG(LOG_FILES,LOG_ERROR)("DOS: File %s is opened but has no psp entry.",shortname);
-               return false;
+               return (Bit16u)false;
            }
            fcb.FileOpen((Bit8u)handle);
-           return true;
+           return handle;
        }
    }

    if (!DOS_OpenFile(shortname,OPEN_READWRITE,&handle)) return false;
    fcb.FileOpen((Bit8u)handle);
-   return true;
+   return handle;
 }

 bool DOS_FCBClose(Bit16u seg,Bit16u offset) {
@@ -1181,11 +1181,12 @@ bool DOS_FCBGetFileSize(Bit16u seg,Bit16
    char shortname[DOS_PATHLENGTH];Bit16u entry;Bit8u handle;Bit16u rec_size;
    DOS_FCB fcb(seg,offset);
    fcb.GetName(shortname);
-   if (!DOS_OpenFile(shortname,OPEN_READ,&entry)) return false;
+   if (!(entry = DOS_FCBOpen(seg, offset))) return false;
    handle = RealHandle(entry);
    Bit32u size = 0;
    Files[handle]->Seek(&size,DOS_SEEK_END);
-   DOS_CloseFile(entry);fcb.GetSeqData(handle,rec_size);
+   fcb.GetSeqData(handle,rec_size);
+   DOS_CloseFile(entry);
    Bit32u random=(size/rec_size);
    if (size % rec_size) random++;
    fcb.SetRandom(random);

FCB和Int 21h /功能23h

1 个答案: