Tandy 1000 HX Technical Reference Manual page 398

Exit Conditions:
AH = 0: light pen switch not activated
AH = 1: light pen values in registers
DH = row of current light pen position
DL = column of current light pen position
CH = raster line (0-199)
BX = pixel column (0-319 or 0-639)
Select Active Page
Select active display page (valid in alpha mode only).
Entry Conditions:
AH = 5
AL = 0 through 7: new page value for modes 0, 1
AL = 0 through 3: new page value for modes 2, 3
AL = 80H: read CRT/CPU page registers
AL = 81H: set CPU page register to value in BL
AL = 82H: set CRT page register to value in BH
AL = 83H: set both CRT and CPU page registers in BH and BL
Exit Conditions:
If bit 7 of AL = 1 upon entry, BH = contents of CRT page
register
BL = contents of CPU page register
Scroll Up
Scroll active page up.
Entry Conditions:
AH = 6
AL = numbers of lines to scroll. The number of lines that will be
left blank at the bottom of the window. (0 means blank
entire window)
CH = row of upper left corner of scroll window
CL = column of upper left corner of scroll window
DH = row of lower right corner of scroll window
DL = column of lower right corner of scroll window
BH = attribute (alpha modes) or color (graphics modes) to be
used on blank line