Routines
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A990: Rotate And Extract Character Code
Used by the routines at ProcessTextCommand, ProcessFormatDataByte and ProcessTextTokenStream.
Input
HL Pointer to bit-packed character data
C Rotation count (bit offset within the data)
Output
A Character code (60-7F)
C Updated rotation count
RotateAndExtractCharacterCode A990 PUSH DE Stash DE on the stack.
Read two bytes from the bit-packed data.
A991 LD D,(HL) Load two bytes from the bit-packed data into DE.
A992 INC HL
A993 LD E,(HL)
A994 DEC HL Decrease HL by one (restore the pointer).
A995 LD A,C Jump to RotateAndExtractCharacterCode_UpdateRotation if no rotation is needed.
A996 AND A
A997 JR Z,RotateAndExtractCharacterCode_UpdateRotation
Rotate the two bytes left by the rotation count.
RotateAndExtractCharacterCode_RotateLoop A999 SLA E Shift E left (with carry) (move bit from high byte to carry).
A99B RL D Rotate D left (move carry bit into low byte).
A99D DEC A Decrease A (the rotation counter) by one.
A99E JR NZ,RotateAndExtractCharacterCode_RotateLoop Jump to RotateAndExtractCharacterCode_RotateLoop if more rotations are needed.
Update the rotation count for the next character extraction.
RotateAndExtractCharacterCode_UpdateRotation A9A0 LD A,C A=C (load the current rotation count).
A9A1 ADD A,$05 A+=05 (add 5 bits for the character we're extracting).
A9A3 CP $08 Jump to RotateAndExtractCharacterCode_Extract if we haven't crossed a byte boundary (A is less than 08).
A9A5 JR C,RotateAndExtractCharacterCode_Extract
A9A7 SUB $08 A-=08 (wrap around within the byte).
A9A9 INC HL Increment HL by one (move to the next byte for the next character).
RotateAndExtractCharacterCode_Extract A9AA LD C,A C=A (store the updated rotation count).
A9AB LD A,D A=D (load the rotated low byte).
A9AC POP DE Restore DE from the stack.
Extract the 5-bit character code from the rotated byte.
A9AD RRCA RRCA three times (shift right by 3 bits to align the 5-bit code).
A9AE RRCA
A9AF RRCA
A9B0 AND %00011111 Keep only bits 0-4 (extract the 5-bit character code).
A9B2 ADD A,$60 A+=60 (convert to ASCII character code range 60-7F).
A9B4 RET Return.
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