; ========================================================================================
; BCDtoHEX.asm - Binary Coded Decimal to Hexadecimal Conversion
; ========================================================================================
; This program converts a BCD (Binary Coded Decimal) value to its equivalent
; hexadecimal representation. BCD stores each decimal digit in 4 bits, so a
; byte can represent two decimal digits (00-99). The program extracts each
; BCD digit and converts the entire BCD number to its decimal equivalent.

	AREA RESET, DATA, READONLY        ; Define a read-only data section for the vector table
	EXPORT __Vectors                  ; Export the vector table for external linking

__Vectors                             ; Start of the vector table
	DCD 0x10001000                    ; Stack pointer initial value (points to top of stack)
	DCD Reset_Handler                 ; Address of the reset handler (program entry point)
	ALIGN                             ; Ensure proper alignment for the next section

	AREA MYCODE, CODE, READONLY       ; Define the code section as read-only
	ENTRY                             ; Mark the entry point of the program
	EXPORT Reset_Handler             ; Export the reset handler function

; ========================================================================================
; Reset_Handler - Main program execution
; ========================================================================================
; Algorithm Overview:
; 1. Load the BCD value from memory (each byte contains two BCD digits)
; 2. Initialize multiplier (R2) to 1 for units place
; 3. Process 3 iterations (for 3 BCD digits in the example)
; 4. In each iteration:
;    a. Extract lowest 4 bits (current BCD digit) using AND
;    b. Multiply digit by current place value and add to result
;    c. Shift BCD value right by 4 bits to get next digit
;    d. Multiply place value by 10 for next iteration
; 5. Result (R4) contains the decimal equivalent of the BCD number

Reset_Handler
	; Step 1: Initialize source pointer and load BCD value
	; R0 points to the BCD value in memory
	LDR R0, =SRR                      ; R0 = address of BCD value

	; Set loop counter for 3 digits (processing 12 bits of the 32-bit value)
	MOV R10, #3                       ; R10 = 3 (number of BCD digits to process)

	; Load the BCD value into R1
	LDR R1, [R0]                      ; R1 = BCD value (e.g., 0x45 = 0100 0101 BCD)

	; Initialize multiplier for place value (1 for units, 10 for tens, etc.)
	MOV R2, #1                        ; R2 = 1 (initial place value multiplier)

	; Step 2: Main BCD to decimal conversion loop
UP
	; Extract the lowest 4 bits (current BCD digit 0-9)
	; AND with 0x0F isolates the least significant nibble
	AND R3, R1, #0x0F                 ; R3 = R1 & 0x0F (extract BCD digit)

	; Multiply digit by place value and accumulate result
	; MLA (Multiply Accumulate) computes: R4 = R2 * R3 + R4
	; This adds the current digit's contribution to the total
	MLA R4, R2, R3, R4                ; R4 = (R2 * R3) + R4

	; Shift BCD value right by 4 bits to access next digit
	LSR R1, #4                        ; R1 = R1 >> 4 (move to next BCD digit)

	; Prepare multiplier for next place value (multiply by 10)
	; This sets up R2 for tens, hundreds, etc. in subsequent iterations
	MOV R5, #0x0A                     ; R5 = 10 (decimal)
	MUL R2, R5                        ; R2 = R2 * 10 (update place value)

	; Decrement loop counter and set condition flags
	SUBS R10, #1                      ; R10 = R10 - 1, set flags for branch condition

	; Branch back to UP if counter is not zero
	BNE UP                            ; If R10 != 0, continue loop

	; Step 3: Program termination
	; Note: The result is stored in R4 but never saved to memory
STOP
	B STOP                            ; Branch to STOP label (infinite loop)

; ========================================================================================
; Data Section - BCD Source Value
; ========================================================================================
; SRR contains a BCD value: 0x45
; In BCD format: 0100 0101 = digits 4 and 5
; Decimal equivalent: 4*10 + 5 = 45
; The program converts this to decimal 45 (though result is never stored)
SRR DCD 0x45                         ; BCD value to be converted

	AREA mydata, DATA, READWRITE      ; Define a read-write data section

; SRC - Additional storage (seems unused in this program)
SRC DCD 0x45                         ; Duplicate of SRR (possibly for testing)

	END                               ; End of the assembly program