Xas99 Teacher

Quick Start

• Prepare a minimal source file with the desired operation.
• Assemble with xas99 to generate a .lst file.
• Review the listing to extract opcode bits and operands.
• (Optional) Assemble with Tawny Petrel and compare encodings.

Common Tasks

Create a minimal source file

Use a consistent scaffolding so addresses are predictable.

    AORG >0000
    DATA WPA    ; WP
    DATA START  ; PC

    AORG >1000  ; workspaces
WPA BSS >20

    AORG >1800
START
    ; put instruction under test here
    IDLE

Create a listing

• Listing file:

~/.local/bin/xas99 -L scratch/<name>.lst -o scratch/ <source>.asm

Extract opcode fields from a listing

• Identify the instruction word (left of the mnemonic).
• Convert the word to binary and mark field boundaries.
• Compare the fields against the ISA encoding.

Example workflow (manual):

1. DIV 5,3 encodes to 3CC5
2. Binary: 0011 1100 1100 0101
3. Fields: 001111 DDDD TT SSSS

Compare with Tawny Petrel

Use the built-in assembler to compare encodings.

dotnet run --project tawny -- asm scratch/<name>.asm -o scratch

Then compare scratch/<name>.lst to the xas99 listing.

Common pitfalls

• Missing/unknown mnemonics: the assembler may treat the mnemonic as a label.
• Operand order: TMS9900 is OP src, dest.
• Addressing modes: use register-only first, then add @, *, or indexed forms.
• Byte vs word forms: SOC vs SOCB, A vs AB.