Logging and terminal stdout

When you run a program on the Spectrum, Spectranext can show its printf/terminal output on your computer—while USB is connected, lines appear in the web tools or wherever you’re connected (see Syncing with Computer).

Viewing logs on Emulators

FuseX Emulator can show debug logs from your program out of the box.

Windows

Select "Machine -> Debug Logs"

Mac OS

To view logs on FuseX on Mac, start Fuse from command line:

/Applications/FuseX.app/Contents/MacOS/FuseX

And any log the Fuse produces will appear in standard output.

Logging architecture

End-to-end, logging looks like this:

1. Your program prints — for example a C program calls printf with libterm linked, which eventually needs to send each character somewhere.
2. libterm turns that into I/O — the library sends those characters with OUT instructions to a fixed I/O port ($043B). Assembly or other code can write the same port directly; you don’t have to use C.
3. Spectranext records it — firmware on the cartridge captures those port writes and appends the bytes to a special file on the device: /stdout (a normal file on the cartridge’s RAM filesystem).
4. Your computer receives it — when USB is connected, another part of the firmware reads /stdout, forwards the text to the host, and clears what was delivered. You see the same text on your PC without doing anything extra.

Writing to /stdout yourself: Anything that ends up in /stdout is treated the same way. There is no separate “logging opcode” in BASIC—you can open that file by name (e.g. "stdout") and PRINT # to it, and it will show up on the host when connected, just like output that came from the I/O port via libterm.

I/O port

For assembly or hand-written I/O, send each byte with OUT (c), a and bc = $043B.

Using libterm from C (z88dk)

Link libterm and use normal printf. For full linker options (including libndos / libspdos and fputc_cons redirect), see the libterm API page.

target_link_options(myprogram PUBLIC
    -pragma-redirect:fputc_cons=spectranext_fputc_cons
)
target_link_libraries(myprogram PUBLIC
    -llibterm
    -lndos
    -llibspectranet.lib
)

#include <stdio.h>

int main(void)
{
    printf("Hello from the Spectrum\n");
    return 0;
}

The libterm printf example under sdk/examples/libterm-printf in the repository is a full CMake project you can copy.

Examples: C, assembly, and BASIC

C

With libterm linked, printf is enough.

#include <stdio.h>

int main(void)
{
    printf("via printf\n");
    return 0;
}

Assembly

Send one character in A through the terminal stdout port:

    ; Character to send in A
    ld      a, 'H'
    ld      bc, $043B
    out     (c), a

    ld      a, 'i'
    out     (c), a

    ld      a, 13       ; newline
    out     (c), a

BASIC

With Spectranext paged in and your filesystem mounted, you can open stdout for writing:

10 %fopen #5, "stdout", "w"
20 PRINT #5; "Hello from BASIC"
30 %close #5

Use stream numbers from 4 upward (#0–#3 are reserved for the system). With USB connected, output shows on your computer like other terminal text.

tip

If stdout isn’t available in your build, use OUT (c), a with bc = $043B, or libterm from C.

Summary

Mechanism	Use case
libterm + printf	C with z88dk
OUT (c), a with bc = $043B	Assembly
%fopen / PRINT # to "stdout"	BASIC
USB connected	Output appears on your computer in real time