MBASIC Compiler CPU Target Options¶

8080 Target¶

The 8080 target works correctly with z88dk:

z88dk.zcc +cpm -clib=8080 --math-mbf32 program.c -o program -create-app

Key flags: - -clib=8080 - Use the classic 8080 library - --math-mbf32 - Include Microsoft Binary Format 32-bit math (needed for floating point)

Why 8080¶

Original CP/M processor - CP/M was designed for the Intel 8080
Maximum compatibility - Works on:
Intel 8080 (original)
Intel 8085 (enhanced 8080)
Zilog Z80 (superset of 8080)
Compatible clones (NEC V20, etc.)
Historical accuracy - MBASIC originally ran on 8080 systems
Wider audience - More systems can run the code

CPU Architecture Differences¶

Intel 8080 (1974)¶

8-bit processor
16-bit address bus (64KB memory)
8-bit I/O ports (IN/OUT instructions)
Basic instruction set
No index registers

Intel 8085 (1976)¶

Binary compatible with 8080
Added SIM/RIM instructions
Integrated clock generator
Can run all 8080 code

Zilog Z80 (1976)¶

Superset of 8080 - runs all 8080 code
Additional registers (IX, IY, alternate set)
More instructions (block moves, bit operations)
16-bit I/O addressing (extended IN/OUT)
More addressing modes

Compiler Options¶

Default (8080)¶

z88dk.zcc +cpm -m8080 program.c -o program

- Most compatible - Works everywhere - Slightly larger/slower code

Z80 Optimized¶

z88dk.zcc +cpm -mz80 program.c -o program

- Uses Z80-specific features - Smaller/faster code - Only works on Z80 systems

Hardware Access Implications¶

I/O Port Access¶

8080 Limitation¶

Only 8-bit port addresses (0-255)
Simple IN/OUT instructions

; 8080 IN instruction
IN port     ; port is 8-bit immediate

; 8080 OUT instruction
OUT port    ; port is 8-bit immediate

Z80 Extension¶

16-bit port addresses (0-65535)
Register-indirect I/O

; Z80 extended IN
LD C,port_low
LD B,port_high
IN A,(C)    ; 16-bit port in BC

; Z80 extended OUT
OUT (C),A   ; 16-bit port in BC

Our Implementation¶

The mb25_hw library provides both versions:

/* Compile with default (8080) */
// Uses self-modifying code for port access
// Limited to ports 0-255

/* Compile with -DUSE_Z80 */
// Uses Z80 extended I/O instructions
// Supports ports 0-65535

Recommendations¶

For Maximum Compatibility¶

Use default 8080 mode: - Historical software preservation - Educational purposes - Wide distribution - Unknown target systems

For Modern Development¶

Consider Z80 mode if: - Target is known Z80 system - Need I/O ports > 255 - Want smaller/faster code - Using Z80-specific hardware

Testing Compatibility¶

Check Generated Assembly¶

# Disassemble to verify 8080 compatibility
z88dk.z88dk-dis program.com > program.asm
grep -i "ix\|iy\|exx\|ldir" program.asm
# If found, code uses Z80-specific instructions

Test on Emulators¶

8080 emulator - Test 8080 compatibility
tnylpo - Z80-based CP/M emulator
SIMH Altair - 8080/Z80 emulation

Library Compatibility¶

The mb25 runtime library is designed for 8080 compatibility:

Component	8080	Z80	Notes
mb25_string	✓	✓	Pure C, no assembly
mb25_hw	✓	✓*	Z80 version with -DUSE_Z80
mb25_math	✓	✓	Uses standard C math
mb25_io	✓	✓	CP/M BDOS calls (portable)

*Hardware functions use conditional compilation for CPU-specific code

Future Enhancements¶

Compiler Flag for CPU Target¶

Could add option to codegen:

class Z88dkCBackend(CodeGenBackend):
    def __init__(self, symbols: SymbolTable, cpu='8080'):
        self.cpu_target = cpu  # '8080' or 'z80'

Runtime CPU Detection¶

Could detect CPU at runtime:

int is_z80() {
    /* Try Z80-specific instruction */
    /* Trap illegal instruction on 8080 */
    /* Return 1 if Z80, 0 if 8080 */
}

Summary¶

Default: 8080 for maximum compatibility
Option: Z80 for enhanced features
Code runs on more systems with 8080 target
Library supports both via conditional compilation
User choice based on target environment