A guide & reference to write PIC Assembly in 2025

this was always confused for anyone to handle-on PIC microcontroller, especially ones that are new to this like me. While I don't want to only write code in C, I want asssembly for smaller size & faster performance. So I solved this complex MPASM -> PIC-AS issue (again) & write it down to share for everyone in 2025.

Why would we waste such simple assembly instruction set like PIC architecture for more complex ones ?

Setup

This was done on A morning of 11th January 2025 with :

• Simulator
• PIC18F45k50
• PIC-AS 2.50
• MPLAB X IDE 6.20 (Tab -> Space)
• Ubuntu 24.10 with kernel 6.11

Then verified & refined on 23th March 2025 to a complete Blink on the same 45k50 chip :

• Fixed PORTA -> LATA : As the same change to C source code, no longer use PORTA but LATA.
• Previous delay wasn't enough so now we have : DELAY * 256 * 256 when using INTOSCIO.
• We can use MOVF REG,W to save value of any register into W-reg.
• We can't compare to zero directly (PICAS v3.00), but rely on 2 methods :
  • CPFSEQ REGISTER : to compare a register value with W-reg.
  • BTFSC STATUS, Z : to test if previous XORLW 0x00 (with W-reg) result in zero.

Key Issues

Key problem during debugging was how the linker didn't link the code properly with previous ORG 0x0000 directive on MPASM.

• PSECT : this is now required for linker to link every parts correctly. And space=[0~3] argument let us access program memory, data memory, reserved, eeprom. Which is quite critical usage on memory.

• BANKSEL : directive to select a memory bank in SRAM, because PIC can only access no more than 256 bytes at once, so it divide memory into banks. Ex: BANKSEL(TRISA) will translate into MOVLB 0xF92 to select TRISA bank.

• Case Sensitive : from PIC-AS, it start to distinct this matter, so MAIN and main are different now.

• #include <xc.inc> : for stuffs like TRISA, LATA...

Debugging

Without PSECT, debugger won't see anything in Program Memory section because your code will never be linked, variables won't be watched :