Master 16-Bit Stack Operations in Real Mode

16-bit stack mechanics

The stack lives at SS:SP. PUSH decrements SP by two and stores a word; POP loads and increments. CALL pushes the return offset; RET pops it. For example, with SS=0x9000 and SP=0xFFFE, a PUSH AX stores AX at physical 0x9FFFE.

Stack frames

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• Alignment: SP should stay even in 16-bit mode
• SS cannot be POP: Load SS with data move, then set SP
• Far CALL: Pushes segment then offset (4 bytes)
• IRET frame: Flags, CS, IP pushed by CPU on interrupts

Stack overflow in boot code silently corrupts nearby sectors; place SS high in conventional memory.

SS:SP initialization

Loading SS does not change SP; always set SP immediately after SS with interrupts disabled if you share the instruction stream with IRQs. A stack growing down from 0xFFFF in segment 0x9000 stays below most boot images loaded at 0x7C00. Far calls push only offset unless you use far proc directives; mismatched near/far return corrupts the stack in subtle ways.

Why for this exercise

You will set up SS:SP and use PUSH/POP, CALL/RET around a small function. This exercise requires computing physical stack addresses and avoiding overlap with your code at 0x7C00.