BUG: SIZE and SIZEOF May Give Incorrect Values for Locals

• Microsoft Macro Assembler for MS-DOS, versions 6.0x and 6.1x

SYMPTOMS

SIZEOF and SIZE drop the high byte of the size for LOCAL variables larger than 255 bytes. The SIZE and SIZEOF for global variables and data types is correct. The TYPE operator works correctly for all variables.

RESOLUTION

You can use the TYPE and LENGTHOF operators to obtain the correct size of local variables.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.x. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

In the following example, structure junk1 is 255 bytes and junk2 is 256 bytes. If local variables are used, SIZE and SIZEOF return 0 (zero) for the size of the local variable j2L. This can be seen in the listing file or in CodeView if the program is run in the debugger. Note that the TYPE operator works correctly for single-element variables. To find the size of array types, use TYPE * LENGTHOF as shown for j3L below.

Sample Code

; assembly options needed: /Zi /Fl /Sa .model large,c .386

junktest PROTO

junk1 struct

    a BYTE 255 DUP (?)

junk2 struct

    a BYTE 256 DUP (?)

.stack 10000 .data

j1 junk1 <> j2 junk2 <>

.CODE

    .STARTUP
        mov ax, sizeof junk1  ; correct sizes here
        mov ax, sizeof j1
        mov ax, size j1
        mov ax, sizeof junk2
        mov ax, sizeof j2
        mov ax, size j2
        invoke junktest
    .EXIT

    LOCAL j1L:junk1     ; correct amount allocated 255 bytes
    LOCAL j2L:junk2     ; correct amount allocated 256 bytes
    LOCAL j3L[256]:BYTE ; correct amount allocated 256 bytes

    mov ax,0
    mov ax, sizeof junk1 ; correct -- 0FFh
    mov ax, sizeof j1L   ; correct -- 0FFh
    mov ax, size j1L     ; correct -- 0FFh
    mov ax, type j1L     ; correct -- 0FFh
    mov ax, sizeof junk2 ; correct -- 0100h
    mov ax, sizeof j2L   ; high byte lost -- returns 00h
    mov ax, size j2L     ; high byte lost -- returns 00h
    mov ax, type j2L     ; OK -- returns 0100h
    mov ax, sizeof j3L   ; high byte lost -- returns 00h
    mov ax, size j3L     ; high byte lost -- returns 00h
    mov ax, type j3L * lengthof j3L ; OK - returns 0100h
    ret

END