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Binary (Base 2) numbers are represented using the WLA-DX format of the "%" symbol followed by a string of binary digits (0 and 1). A valid binary number representation would be: %11100110.

I.2.2 Bit and Byte Order

Bits are numbered starting from right to left, (i.e., least-significant to most-significant.) All bytes consist of eight bits: 0 through 7.

The SMS and GG machines are both based on Zilog's Z80 microprocessor. This microprocessor is a "little-endian" machine, meaning that bytes of a word are numbered starting from the least-significant byte. As a result, the SMS and GG machines are also little-endian. The least-significant byte in a word will always be stored to memory first, at the lower address, followed by the most-significant byte at the higher address.

Diagrams of data structures in memory will have lower memory addresses towards the bottom of the page, with higher addresses growing towards the top of the page.

I.2.3 Reserved Bits

In certain register layout descriptions, various bits will be marked as "Reserved." All reserved bits are implicitly RESET (0). Software should never attempt to SET these bits (to 1). Follow the guidelines below:

Do not depend on the states of any reserved bits when testing the values of defined register bits. Mask out the reserved bits when testing.
Do not depend on the states of any reserved bits when storing them to memory or another register.
Do not depend on the ability to retain information written into any reserved bits.
Always load the reserved bits as zeros.

I.2.4 Instruction Operands

The main processor used in the SMS and GG machines is the Zilog Z80 microprocessor. The symbolic representation of instructions is taken directly from Z80 assembly language. The format is shown below:

label: mnemonic argument1, argument2

where:

A label is an identifier followed by a colon (:).
A mnemonic is a reserved name for a class of instruction opcodes which have the same function.
The operands argument1, argument2 are optional. There may be zero, one, or two arguments, depending on the instruction. When present, they are either literals, or data identifiers. The data identifiers are either reserved register names, or are assigned to data items declared elsewhere in the source file.

When two operands are present, the right operand is the source and the left is the destination.

For example:

LOOP1:  LD HL, STARTVALUE

In this example, LOOP1 is a label, LD is the mnemonic identifier for the opcode, HL is the destination operand, and STARTVALUE is the source operand.

I.3 Revision History

I.4 Acknowledgements

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S8-Dev Standards Committee

I.1 About this Document

I.2 Document Conventions

I.2.1 Hexadecimal and Binary Numbers

I.2.2 Bit and Byte Order

I.2.3 Reserved Bits

I.2.4 Instruction Operands