Prism provides the ANOVA table so you can compare results with other programs.

The ANOVA table starts with the  variation of the outcome (Y) variable, without considering the other variables or the regression. This is calculated as the sum of the square of the difference between each Y value and the mean of all the Y values. This is reported as the total sum of squares in the bottom row of the ANOVA table.

Then it computes the sum of the square of the discrepancy (difference) between the actual Y values and the Y values predicted by the regression model. This is the sum of squares of the residuals.

The difference between these two values is the sum of squares of the regression.

The ANOVA table computes a F ratio, which is used to compute a P value. This P value tests the null hypothesis that the regression model is entirely useless so the predictions of the regression model are no better than just predicting that each Y value equals the mean of all Y values. It is only worth looking at the other regression results when this P value is small.