The stack plays an important role in all microprocessors. It holds data temporarily and stores the return addresses used by procedures. The stack memory is an LIFO (last-in, first-out) memory, which describes the way that data are stored and removed from the stack. Data are placed onto the stack with a PUSH instruction and removed with a POP instruction. The CALL instruction also uses the stack to hold the return address for procedures and a RET (return) instruction to remove the return address from the stack.
The stack memory is maintained by two registers: the stack pointer (SP or ESP) and the stack segment register (SS). Whenever a word of data is pushed onto the stack [see Figure 3–17(a)], the high-order 8 bits are placed in the location addressed by SP – 1. The low-order 8 bits are placed in the location addressed by SP – 2. The SP is then decremented by 2 so that the next word of data
is stored in the next available stack memory location. The SP/ESP register always points to an area of memory located within the stack segment. The SP/ESP register adds to SS * 10H to form the stack memory address in the real mode. In protected mode operation, the SS register holds a selector that accesses a descriptor for the base address of the stack segment.
Whenever data are popped from the stack [see Figure 3–17(b)], the low-order 8 bits are removed from the location addressed by SP. The high-order 8 bits are removed from the location addressed by SP + 1. The SP register is then incremented by 2. Table 3–11 lists some of the PUSH and POP instructions available to the microprocessor. Note that PUSH and POP store or retrieve words of data—never bytes—in the 8086 through the 80286 microprocessors. The 80386 and above allow words or doublewords to be transferred to and from the stack. Data may be pushed onto the stack from any 16-bit register or segment register; in the 80386 and above, from any 32-bit extended register. Data may be popped off the stack into any register or any segment register except CS. The reason that data may not be popped from the stack into CS is that this only
changes part of the address of the next instruction. In the Pentium 4 or Core2 operated in 64-bit mode, the 64-bit registers can be pushed or popped from the stack, but they are 8 bytes in length.
The PUSHA and POPA instructions either push or pop all of the registers, except segment registers, onto the stack. These instructions are not available on the early 8086/8088 processors. The push immediate instruction is also new to the 80286 through the Core2 microprocessors. Note the examples in Table 3–11, which show the order of the registers transferred by the PUSHA and POPA instructions. The 80386 and above also allow extended registers to be pushed or popped. The 64-bit mode for the Pentium 4 and Core2 does not contain a PUSHA or POPA instruction.
Example 3–15 lists a short program that pushes the contents of AX, BX, and CX onto the stack. The first POP retrieves the value that was pushed onto the stack from CX and places it into AX. The second POP places the original value of BX into CX. The last POP places the value of AX into BX.
