PRB: INVOKE Pushes SS to Convert NEAR PTR to FAR PTR

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

When an application passes a parameter of type NEAR PTR to a routine that expects an argument of type FAR PTR, the INVOKE directive converts the NEAR PTR to a FAR PTR by pushing the SS segment register on the stack. The assembler does not issue any warnings when it applies this conversion. In a situation where SS does not equal DS, as in a Windows dynamic-link library (DLL), INVOKE may generate incorrect code.

CAUSE

When the assembler converts a NEAR PTR to a FAR PTR, it uses the SS segment register when it has no other information about the appropriate segment value.

RESOLUTION

When your application passes a NEAR PTR parameter to a function that expects a FAR PTR, specify the segment register to use in the conversion by adding the appropriate segment prefix (cs:, ds:, es:, or ss:) to the parameter. The code example below demonstrates this method.

Sample Code

; Assemble options needed: none

.MODEL small, C, farstack

func1 PROTO arg1:FAR PTR BYTE

.CODE func2 PROC, arg2:NEAR PTR BYTE

    INVOKE func1, arg2         ; This pushes ss:arg2
    ret

func3 PROC, arg2:NEAR PTR BYTE

    INVOKE func1, ds:arg2      ; This pushes ds:arg2
    ret

END