; RS232_String_Echo_2e.asm

; 2025-12-08 prefontDon@gmail.com

;***************************************************************************
; PROGRAM:	RS232_String_Echo_2e.asm
; PURPOSE:	Initializes UART, prints a string, then echoes user keypresses.
;***************************************************************************
#define 	EQU .equ

; UART Register Definitions:
UART_R0:	EQU $00   	; RBR/THR (00 + 01).
UART_R2:	EQU $02   	; Interrupts, FCR.
UART_R3:	EQU $03   	; LCR.
UART_R4:	EQU $04 	; MCR.
UART_R5:	EQU $05   	; LSR.
UART_R6:	EQU $06   	; MSR.
UART_R7:	EQU $07		; Scratch register.

EOS:		EQU $00		; End of string marker.
CR:         EQU $0d    	; Carriage return.
LF:         EQU $0a    	; Line feed.
space:      EQU $20    	; Space.
tab:        EQU $09    	; Tabulator.	

ROM_START	EQU $0000
ROM_END		EQU $7FFF	; 32KB ROM.
RAM_START	EQU $8000
RAM_END		EQU $FFFF	; 32KB RAM.

.ORG $0000

		LD SP,	RAM_END			; Stack Pointer at top or RAM.
		
		LD	HL,	0
		LD	BC,	0
		CALL UART_DELAY_100MS	; UART delay needed to get ready.
		JP 	UART_INIT			; Initialize UART.

;***************************************************************************
;UART DELAY:
;***************************************************************************
; Z80 Nested Loop Delay Routine (Uses B and C registers)
; CPU Clock: 1 MHz (1 T-State = 1 us)
; Total Delay: Approximately 102 milliseconds (ms)
UART_DELAY_100MS:   
		PUSH	BC
		LD      B, 25   		; Outer Loop Setup: load B with the outer counter (25 repetitions).       
OUTER_LOOP:
		LD      C, 255  		; Inner Loop Setup:  Load C with the inner counter (255 repetitions).  
INNER_LOOP:				
		; --- Inner Loop Execution ---
		DEC     C       		; Decrement the inner counter (4 T-states).
		JR      NZ, INNER_LOOP 	; Jump back if C is not zero (7/12 T-states).
        ; --- Outer Loop Execution ---
		DJNZ    OUTER_LOOP  	; Decrement B, and jump back to OUTER_LOOP if B is not zero (8/13 T-states).
		POP	BC
		RET                 	; Return from the subroutine.
		
;***************************************************************************		
; 1. INITIALIZATION SEQUENCE:
;***************************************************************************
UART_INIT:
        LD A,	$80				; Set DLAB=1 for configuration.
		OUT (UART_R3), A		
			
		LD A,	$0C				; Baud Rate Divisor LSB (9600bps).
		OUT (UART_R0), A		
		
		LD A,	$00				; Baud Rate Divisor MSB.
		OUT (UART_R0 + 1), A	; Note: UART_R1 = UART_R0 + 1.
		
		LD A,	$07				; Value 07h: Enable FIFOs, Clear Rx FIFO, Clear Tx FIFO.
		OUT (UART_R2), A		; Write to FCR (UART_R2). *FCR reset*.	
		
		LD A,	$03				; Set 8-N-1, DLAB=0 (end of configuration).
		OUT (UART_R3), A	    
		
		JP STRING_LOOP
		
;***************************************************************************		
; 2. STRING PRINTING CALL (Changed from 10 string printings to one):
;***************************************************************************
;STRING_LOOP:
		;LD B,	10
;STRING_REPEAT:
		;CALL	STRING_START
		;DEC	B
		;JR	NZ, STRING_REPEAT
		;JP MAIN_ECHO_LOOP
		
;STRING_START:
		;LD HL,	RS232_Echo		; Load address of string
		;CALL 	PRINT_STR_ONCE	; Print the string
		;RET
		
STRING_LOOP:
		LD HL,	RS232_Echo		; Load address of string
		;CALL 	PRINT_STR_ONCE	; Print the string
		;JP	STRING_LOOP
		JP	PRINT_STR_ONCE	; Print the string
		
;***************************************************************************
; PRINT_STR_ONCE Subroutine:
; This subroutine uses CALL/RET but is only executed ONCE, minimizing
; the risk of the THRE lockup compared to calling it repeatedly.
; Registers used: A, HL. Registers B, C, D, E are preserved.
;***************************************************************************
PRINT_STR_ONCE:
		PUSH AF					; Save A and Flags
		;PUSH BC
		;PUSH HL				; Save the return pointer HL (not strictly needed, but good practice)
		
PRNT_LOOP:
		LD		A,(HL)			; Get char from HL
		CP		EOS				; Test for End Of String (00h)
		JP		Z, PRNT_END		; Jump if EOS found
		LD	B,	A
		
		; --- TRANSMIT SINGLE CHARACTER ---
TX_READY_WAIT:
		IN A,	(UART_R5)		; Check LSR (THRE bit 5)
		BIT 5,	A				
		JP Z,	TX_READY_WAIT	; Wait until ready
		LD A, 	B
		OUT (UART_R0), A		; Send character
		INC		HL				; Increment string pointer
		JP		PRNT_LOOP
		
PRNT_END:
		;POP HL					; Restore HL
		;POP BC
		POP AF					; Restore A and Flags
		;RET
		JP MAIN_ECHO_LOOP		; Use this for single iteration.
		;JP	STRING_LOOP			; Use this for continuous testing.
;***************************************************************************		
; 3. MAIN ECHO LOOP:
;***************************************************************************
MAIN_ECHO_LOOP:
		; --- GET CHARACTER (RX) ---
GET_CHAR_UART:   
		IN A,	(UART_R5)		; Check LSR (DR bit 0).
		BIT 0,	A				
		JP Z,	GET_CHAR_UART	; Loop until char is ready.
		
		IN A,	(UART_R0)		; Read character into A (Clears DR flag).
		LD B,	A				; Store character into B Register for safe keeping.
			
		; --- SEND CHARACTER (TX) ---
SEND_CHAR_UART:   
		IN A,	(UART_R5)		; Check LSR (THRE bit 5).
		BIT 5,	A				
		JP Z,	SEND_CHAR_UART	; Loop until Transmitter Holding Register (THRe) is Empty.
		
		LD A,	B		    	; Get character from B Register.
		OUT (UART_R0), A		; Send character.
			
		JP MAIN_ECHO_LOOP		; Continue the loop of echoing user keypresses.

;*************************************************************************	
RS232_Echo:
			.DB "RS232_String_Echo_2e.bin", $0D,$0A,$00
;*************************************************************************				
.END