Contributor: TREVOR CARLSON
{
TF>How does one alter a DOS environment variable in PASCAL and have the change
TF>reflected after the program terminates, leaving the user in DOS, and the use
TF>types SET? This has been bugging me for a while. I know that there are two
TF>copies of the environment and I need to access the top one, but I don't know
TF>how.
The following example shows how to change the prompt: }
function MastEnvSeg(var Envlen: word): word;
{-returns the master environment segment }
var
mcb,temp,handle : word;
lastmcb : boolean;
begin
MastEnvSeg := 0;
Envlen := 0;
handle := MemW[0: $ba]; {-2ドルe * 4 + 2}
{-The interrupt vector 2ドルe points to the first paragraph of
allocated to the command processor}
mcb := pred(handle);
{-mcb now points to the memory control block for the command processor}
repeat
temp := Mcb+MemW[Mcb:3]+1;
if (Mem[temp:0] = 4ドルd) and (MemW[temp:1] = handle) then begin
lastmcb := false;
mcb := temp;
end
else
lastmcb := true;
until lastmcb;
EnvLen := Mem[Mcb:3] shl 4;
MastEnvSeg := succ(Mcb);
end;
procedure InitNewPrompt;
{-set up a new prompt for shelling to dos}
type
_2karray = array[1..2048] of byte;
SegPtr = ^_2karray;
const
NewPrompt : string =
('PROMPT=Type EXIT to return to program$_$p$g'+#0);
var
EnvSegment,
NewEnvSeg : word;
PtrSeg,
NewEnv : SegPtr;
begin
EnvSegment := memw[prefixseg:2ドルC];
{-this gets the actual starting segment of the current program's env}
PtrSeg := ptr(pred(EnvSegment),0);
{-The segment of the program's MCB - (Memory control block) }
getmem(NewEnv,1072+length(NewPrompt));
{-Allocate heap memory and allow enough room for a dummy mcb }
if ofs(NewEnv^) 0 then
NewEnvSeg := seg(NewEnv^) + 2
else
NewEnvSeg := succ(seg(NewEnv^));
{-Force the new environment to start at paragraph boundary}
move(PtrSeg^,mem[pred(NewEnvSeg):0],16);
{-copy the old mcb and force to paragraph boundary}
memw[pred(NewEnvSeg):3] := (1072+length(NewPrompt)) shr 4;
{-Alter the environment length by changing the dummy mcb}
move(NewPrompt[1],memw[NewEnvSeg:0],length(NewPrompt));
{-install new prompt}
memw[prefixseg:2ドルC] := NewEnvSeg;
{-let the program know where the new env is}
move(mem[EnvSegment:0],mem[NewEnvSeg:length(NewPrompt)],1024);
{-shift the old env to the new area}
end;