.Z80
ASEG      ; absolute addressing
ORG  100h ; skip CP/M

CR   EQU  0Dh       ; carriage return
LF   EQU  0Ah       ; new line
BDOS EQU  0005h     ; BDOS entry point
PSTR EQU  09h       ; print string
CRED EQU  01h       ; read console

OLED EQU  50h  ; base addr of OLED card
ORST EQU  04h  ; reset command (H)
OBUF EQU  02h  ; chip select (L), buffer (H)
ODB  EQU  01h  ; display byte (H)
OCMD EQU  00h  ; command (L)

; 4 rows of display
OROWA EQU  0B0h  ; select top row
OROWB EQU  0B1h  ; select second row
OROWC EQU  0B2h  ; select third row
OROWD EQU  0B3h  ; select bottom row

OCOL0 EQU  04h  ; left most column
OSELC EQU  10h  ; select column command
OCOLL EQU  80h  ; column count: 128 columns

; 128 columns from 4 to 131
; 1Xh, 0Yh - XY = 04h to 83h

; MAIN
CALL OINIT     ; init OLED
CALL OCLS      ; clear screen
CALL PROMPT    ; print prompt

CALL OCSTART   ; go to start 
CALL READLINE  ; read row A
CALL READLINE  ; read row B
CALL READLINE  ; read row C
CALL READLINE  ; read row D

RET            ; exit to CP/M


READLINE:
     LD   D,OCOLL   ; number of columns
READLINE1:
     PUSH DE
     CALL READHEX   ; read hex input from console
     CALL ODATA     ; write to OLED
     POP  DE
     DEC  D         ; count donw columns
     JP   NZ,READLINE1  ; next column 
     RET

OINIT:
     CALL ORESET    ; reset OLED
     LD   HL,OINITSEQ ; commands to send
OINITLOOP:
     LD   A,(HL)    ; load command byte to A
     CP   0FFh      ; check if we are done
     RET  Z         ; marker reached
     CALL OCOMMAND  ; send command from A
     INC  HL        ; next command
     JR   OINITLOOP ; send next command 

ORESET:
     LD   C,OLED    ; base I/O address
     LD   B,ORST OR OBUF ; RST H, /CS H, DB L
     OUT  (C),A     ; write any A, to (C) + B
     REPT 4
          NOP       ; delay
     ENDM
     LD   B,OBUF    ; clear RST
     OUT  (C),A     ; send
     REPT 20
          NOP       ; long delay
     ENDM
     RET

OCLS:
     CALL OCSTART   ; set start
     CALL OCLSROW   ; clear row A
     CALL OCLSROW   ; clear row B
     CALL OCLSROW   ; clear row C
     CALL OCLSROW   ; clear row D
     RET

; set cursor to start 
OCSTART:
     LD   A,OSELC   ; select column
     CALL OCOMMAND
     LD   A,OCOL0   ; column 0 
     CALL OCOMMAND
     LD   A,OROWA   ; select first row
     CALL OCOMMAND
     RET

OCLSROW:
     ;XOR  A        ; display byte to write (0)
     LD   A,0FFh    ; turn on all pixels
     LD   D,OCOLL   ; count columns in D
OCLSROW1:
     CALL ODATA     ; write A to displey
     DEC  D         ; next column
     JR   NZ,OCLSROW1
     RET

; A = command to send to OLED
OCOMMAND:
     LD   C,OLED    ; base I/O
     LD   B,OCMD    ; send command
     OUT  (C),A     ; send
     REPT 4
          NOP
     ENDM
     LD   B,OCMD OR OBUF
     OUT  (C),A     ; send
     REPT 10
          NOP
     ENDM
     RET

; Write to OLED memory
; A = graphic byte to dispaly memory
ODATA:
     LD   C,OLED    ; base I/O
     LD   B,ODB     ; select Data Byte mode
     OUT  (C),A     ; output byte
     REPT 4
          NOP
     ENDM
     LD   B,OBUF OR ODB
     OUT  (C),A     ; output byte
     NOP
     NOP
     RET

; Read byte from console
READ:
     LD   C,CRED    ; read from console
     CALL BDOS      ; call CP/M
     RET            ; A=CHAR

; Reads 4 bit value (0-F)
READNIB:
     CALL READ
     LD   B,41h     ; compare with 'A' (10)
     CP   B
     JP   NC,RNB1   ; A>=B
     SUB  30h       ; sub '0'
     RET
RNB1:
     SUB  41h-10    ; sub 'A'-10 
     AND  0Fh       ; clear to 4 bits so 'a-f' also work
     RET

; Reads two bytes (0-F) from console
; A = byte read
READHEX:
     CALL READNIB   ; read MSD nibble
     SLA  A         ; shift 4 bits
     SLA  A
     SLA  A
     SLA  A
     LD   B,A       ; B=A
     PUSH BC        ; save B
     CALL READNIB   ; read LSD nibble
     POP  BC        ; restore B
     OR   B         ; OR B to A
     RET

PROMPT:
     LD   C,PSTR    ; load function number
     LD   DE,PROMPTMSG    ; load string addr
     CALL BDOS      ; call CP/M
     RET

PROMPTMSG: 
     DB   'OLED:>', CR, LF,'$' ; string terminated with $

; Initialization sequence
; 081h,080h - set contrast (080h full)
; 020h,000h - set horizontal window wrapping
; 021h,004h,083h - window full width
; 022h,000h,003h - window full height
OINITSEQ:
     DB   0AEh,0D5h,0A0h,0A8h
     DB   01Fh,0D3h,000h,0ADh
     DB   08Eh,0D8h,005h,0A1h
     DB   0C8h,0DAh,012h,091h
     DB   03Fh,03Fh,03Fh,03Fh
     DB   081h,070h,0D9h,0D2h
     DB   0DBh,034h,0A6h,0A4h
     DB   0AFh,020h,000h,021h
     DB   004h,083h,000h,003h
     DB   0FFh ; end marker

END