The contents of the register pair qq are pushed to the external memory last-in, first-out (LIFO) stack. The Stack Pointer (SP) Register pair holds the 16-bit address of the current top of the Stack. This instruction first decrements SP and loads the high-order byte of reg- ister pair qq to the memory address specified by the SP. The SP is decremented again and loads the low-order byte of qq to the memory location corresponding to this new address in the SP. The operand qq identifies register pair BC, DE, HL, or AF, assembled as follows in the object code: Pair qq BC 00 DE 01 HL 10 AF 11
If the AF Register pair contains 2233h and the Stack Pointer contains 1007h, then upon the execution of a PUSH AF instruction, memory address 1006h contains 22h, memory address 1005h contains 33h, and the Stack Pointer contains 1005h.