Incorrect description of SDB behavior in TrueType instruction set

[T-Sawamoto-X]

Description

Document

TrueType Instruction Set - SDB

Problem

The "Pops" field for the SDB (Set Delta_Base) instruction is currently listed as empty. However, according to the TrueType specification and actual interpreter behavior, SDB pops one value from the stack(the new delta_base value).

Evidence

• Instruction Set - TrueType Reference Manual - Apple Developer (Apple)
• FreeType implementation: SDB is handled with args[0] in ttinterp.c / Ins_SDB.

Suggested Fix

Update the "Pops" field for SDB to "n: value for the delta_base (uint32)".

Additional Notes

This correction improves clarity for font developers and tool implementers who rely on accurate stack behavior for instruction sequencing and debugging.

Page URL

https://learn.microsoft.com/en-us/typography/opentype/spec/tt_instructions?source=recommendations

Content source URL

https://github.com/MicrosoftDocs/typography/blob/live/typographydocs/opentype/spec/tt_instructions.md