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16.3 Sharing Data Segments Among Mixed Code Segments

Because the choice of operand size and address size is defined in code segments and their descriptors, data segments can be shared freely among both USE16 and USE32 code segments. The only limitation is the one imposed by pointers with 16-bit offsets, which can only point to the first 64 Kilobytes of a segment. When a data segment that contains more than 64 Kilobytes is to be shared among USE32 and USE16 segments, the data that is to be accessed by the USE16 segments must be located within the first 64 Kilobytes.

A stack that spans addresses less than 64K can be shared by both USE16 and USE32 code segments. This class of stacks includes:

  • Stacks in expand-up segments with G=0 and B=0.
  • Stacks in expand-down segments with G=0 and B=0.
  • Stacks in expand-up segments with G=1 and B=0, in which the stack is contained completely within the lower 64 Kilobytes. (Offsets greater than 64K can be used for data, other than the stack, that is not shared.)
The B-bit of a stack segment cannot, in general, be used to change the size of stack used by a USE16 code segment. The size of stack pointer used by the processor for implicit stack references is controlled by the B-bit of the data-segment descriptor for the stack. Implicit references are those caused by interrupts, exceptions, and instructions such as PUSH, POP, CALL, and RET. One might be tempted, therefore, to try to increase beyond 64K the size of the stack used by 16-bit code simply by supplying a larger stack segment with the B-bit set. However, the B-bit does not control explicit stack references, such as accesses to parameters or local variables. A USE16 code segment can utilize a "big" stack only if the code is modified so that all explicit references to the stack are preceded by the address-size prefix, causing those references to use 32-bit addressing.

In big, expand-down segments (B=1, G=1, and E=1), all offsets are greater than 64K, therefore USE16 code cannot utilize such a stack segment unless the code segment is modified to employ 32-bit addressing . (Refer to Chapter 6 for a review of the B , G, and E bits .)


up: Chapter 16 -- Mixing 16-Bit and 32 Bit Code
prev: 16.2 Mixing 32-Bit and 16-Bit Operations
next: 16.4 Transferring Control Among Mixed Code Segments