Thank you for your reply.

How do I do that? (choose pins) Just by putting them in SPI()??

And also I am surprised at the double distribution for SPI2... it does not indicate that in the pin map

Also, I read about PB_6 in the homepage this question is posted in.(https://developer.mbed.org/platforms/ST-Nucleo-F091RC/) (read the blue box called "Information")

Like this for example.
Note that I did not try it on the board, I only compiled.
This is based on the original mbed SPI example

#include "mbed.h"
 
SPI spi(PB_4, PB_5, PB_3); // mosi, miso, sclk
DigitalOut cs(PA_15);
 
int main() {
    // Chip must be deselected
    cs = 1;

    // Setup the spi for 8 bit data, high steady state clock,
    // second edge capture, with a 1MHz clock rate
    spi.format(8,3);
    spi.frequency(1000000);
 
    // Select the device by seting chip select low
    cs = 0;
 
    // Send 0x8f, the command to read the WHOAMI register
    spi.write(0x8F);
 
    // Send a dummy byte to receive the contents of the WHOAMI register
    int whoami = spi.write(0x00);
    printf("WHOAMI register = 0x%X\n", whoami);
 
    // Deselect the device
    cs = 1;
}

Regarding the CS pin, it seems like mbed does not use the hardware NSS function. Any GPIO can be used to drive the CS pin. This allows to drive several SPI devices by using several GPIOs, one for each device.

Any pin can be used as CS when you are SPI Master. The pin must be set and reset explicitly by your software like in the example above. However, when you use the mbed SPISlave class the CS, which is now an input pin, must be assigned to the correct NSS pin for the SPI hardware engine that you have selected.

Thank you!