Build a Flat GDT for Protected Mode

Flat GDT layout

The GDT holds segment descriptors describing base, limit, and access rights. A flat model maps code and data to the full 4 GB space with base 0 and limit 4 GB using granularity bit. For example, entry 1 (selector 0x08) is ring-0 code and entry 2 (selector 0x10) is ring-0 data.

Descriptor template

nasmCopy
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• Access byte: Present (P), DPL, executable, readable/writable
• Granularity: Set 4K pages for limit → full 32-bit span
• Null descriptor: Index 0 must be unused
• Selector = index << 3 (plus RPL bits)

Access byte pitfalls

Setting the D/B bit in the granularity byte marks a code segment as 32-bit default operand size. Data segments need the writable bit set or stores fault. Limit 0xFFFFF with granularity 4K yields a 4 GB span with base 0, which is the textbook flat model. Reload SS with the data selector after entering protected mode before any push instructions execute.

Why for this exercise

You will author a three-entry GDT and load it with LGDT before mode switch. This exercise requires correct limit/base encoding and selectors 0x08/0x10 for code and data segments.