PC-E220-Tools/doc/base64.md

Implementing BASE64 in Z80 Assembler

About

Goal of this little project is to encode and decode a byte buffer in memory. The project defines the following requirements.

1. The function takes the pointer to an input buffer as a parameter on the stack? Can also be in one of the automatic variables.
2. Another variable contains the address of the 2nd buffer.
3. We need to find a way to get the addresses of a DIM variable on the stack, in order to call it from BASIC.
4. Implement a subroutine for encoding a string.
5. Implement a subroutine for decding a string.

Problem No. 1: Splitting 3-8bit values into 4 6-bit-values.

Example:

We want to convert the string "ABC". In binary the values are as follows.

• 01000001
• 01000010
• 01000011

The first step is to reduce the first byte to 6 bytes:

        LD A,(HL)
        SLA,2
        PUSH A