Routines
Prev: 43355 Up: Map Next: 43445
43408: 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 (96-127)
C Updated rotation count
RotateAndExtractCharacterCode 43408 PUSH DE Stash DE on the stack.
Read two bytes from the bit-packed data.
43409 LD D,(HL) Load two bytes from the bit-packed data into DE.
43410 INC HL
43411 LD E,(HL)
43412 DEC HL Decrease HL by one (restore the pointer).
43413 LD A,C Jump to RotateAndExtractCharacterCode_UpdateRotation if no rotation is needed.
43414 AND A
43415 JR Z,RotateAndExtractCharacterCode_UpdateRotation
Rotate the two bytes left by the rotation count.
RotateAndExtractCharacterCode_RotateLoop 43417 SLA E Shift E left (with carry) (move bit from high byte to carry).
43419 RL D Rotate D left (move carry bit into low byte).
43421 DEC A Decrease A (the rotation counter) by one.
43422 JR NZ,RotateAndExtractCharacterCode_RotateLoop Jump to RotateAndExtractCharacterCode_RotateLoop if more rotations are needed.
Update the rotation count for the next character extraction.
RotateAndExtractCharacterCode_UpdateRotation 43424 LD A,C A=C (load the current rotation count).
43425 ADD A,5 A+=5 (add 5 bits for the character we're extracting).
43427 CP 8 Jump to RotateAndExtractCharacterCode_Extract if we haven't crossed a byte boundary (A is less than 8).
43429 JR C,RotateAndExtractCharacterCode_Extract
43431 SUB 8 A-=8 (wrap around within the byte).
43433 INC HL Increment HL by one (move to the next byte for the next character).
RotateAndExtractCharacterCode_Extract 43434 LD C,A C=A (store the updated rotation count).
43435 LD A,D A=D (load the rotated low byte).
43436 POP DE Restore DE from the stack.
Extract the 5-bit character code from the rotated byte.
43437 RRCA RRCA three times (shift right by 3 bits to align the 5-bit code).
43438 RRCA
43439 RRCA
43440 AND %00011111 Keep only bits 0-4 (extract the 5-bit character code).
43442 ADD A,96 A+=96 (convert to ASCII character code range 96-127).
43444 RET Return.
Prev: 43355 Up: Map Next: 43445