Tandy 1000 SL Technical Reference Manual page 537

Software
Write Dot
Write a pixel (dot).
Entry Conditions
AH
=
OCH
DX
=
rownumber
CX
=
columnnumber
AL
=
color value (When Bit 7 of AL is set, the resultant
color value of the dot is the exclusive OR of the current
dot
color value and the value in AL.)
Read Dot
Read a pixel (dot).
Entry Conditions
AH
=
ODH
DX
=
rownumber
CX
=
columnnumber
Exit Conditions
AL
=
color value of dot read
16.