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The mixing-length theory for the convection, as it is used in the ATLAS9 code (Kurucz, 1993a), is summarized and discussed. We investigated the effect of the modication called \approximate overshooting on the model structure of the Sun andofstarswithTeff includedbetween4000Kand8500K,logg included between 2.5 and 4.5, and metallicities (M/H)=0.0 and (M/H)= 3:0.WefoundthattheKuruczsolarmodel(SUNK94) with the \overshooting option switched on reproduces more observations than that without \overshooting. In the H and H regions no solar model is able to reproduce the level of the true continuum deduced from high-resolution observations ab- solutelycalibrated.At486nmthecomputedcontinuumisabout 6.6% higher than that inferred from the observed spectrum. We found that the largest effect of the \approximate overshooting on the model structure occurs for models withTeff>6250 K and it decreases with decreasing gravity. The differences in (b y), (B V), and (V K) indices computed from models with the \overshooting option switched on and off, correspond to Teff differenceswhichmayamountupto180K,100K,60Krespec- tively. The differences in Teff from Balmer proles may amount up to 340 K and they occur also forTeff<6250 K down to about 5000 K. The c1 index yields gravity differences logg as a functionoflogg which,foreachTeff,growtoamaximumvalue. The maximum logg decreases with increasing temperatures and ranges, for solar metallicity, from 0.7 dex at logg=0.5 and Teff=5500 K to 0.2 dex at logg=4.5 and Teff=8000 K. This be- haviour does not change for (M/H) = 3:0. Comparisons with the observations indicate that model parameters derived with different methods are more consistent when the \overshooting option is switched off (NOVER models), except for the Sun. In particular for Procyon, Teff and logg from NOVER models are closer to the parameters derived from model independent methods than are Teff and logg derived from the Kurucz (1995) grids. However, no model is able to explain the whole observed spectrumofeithertheSunorProcyonwithauniqueTeff,regard- less of whether the \overshooting option is switched on or off. Independently of the convection option, the largest differences