Web 51 P-Code Interpreter

Part 2 - P-Instructions for arithmetics and program branches

Connection of P-Code Interpreter with individual P-Instructions can be demonstrated e.g. on the math library. Skeleton of a P-Instruction can be shown on the following example:
;
; *** PCMPBI *** compare STRING (cpu IDATA mem) with BYTE constant ***
; pcmpbi buffer_addr, byteconst
;       R7,  R5
pcmpbi: SETB zflag
        MOV  A,R7
        MOV  R0,A
        CLR  C
        MOV  A,@R0
        SUBB A,R5
        MOV  cflag,C
        JZ   pcmpbi0
        CLR  zflag
pcmpbi0:RET
Input parameters are passed in R7 (parameter 1) and R5 (parameter 2) as detailed in the previous part. This procedure compares two numbers, one specified indirectly through R7, the other directly in R5. Comparison results are stored into global bit variables zflag and cflag. In the I51 mnemonic, the procedure does: zflag = (@R7 == R5), cflag = (@R7 < R5). Program can use these bit variables (defined in the main P-Code Interpreter module - Pcode.asm) to branch. Procedure is terminated with RET instruction.

Contents of LIBW - Library of math functions

*instruction*	*parameters*	*description*	*operation*	*flags*
pcmpbi	(B)a, (B)imm	Compare Byte to Immediate	(BYTE)@a - (BYTE)imm	zflag, cflag
pmovbi	(B)a, (B)imm	Mov Immediate to Byte	(BYTE)@a = (BYTE)imm
pmovb	(B)a, (B)b	Mov Byte to Byte	(BYTE)@a = (BYTE)@b
pcmpwi	(B)a, (W)imm	Compare Word to Immediate	(WORD)@a - (WORD)imm	zflag, cflag
pbitwi	(B)a, (W)imm	Test bit Immediate Word	(WORD)@a AND (WORD)imm	zflag
pmovwi	(B)a, (W)imm	Mov Immediate to Word	(WORD)@a = (WORD)imm
paddwi	(B)a, (W)imm	Add Immediate to Word	(WORD)@a += (WORD)imm
psubwi	(B)a, (W)imm	Subbtract Immediate Word	(WORD)@a -= (WORD)imm
pxorwi	(B)a, (W)imm	XOR Immediate Word	(WORD)@a = (WORD)@a XOR (WORD)imm
pandwi	(B)a, (W)imm	AND Immediate Word	(WORD)@a = (WORD)@a AND (WORD)imm
pshiftw	(B)a, (B)n	Shift Word n-bit Left/Right	if(n>0){(WORD)@a << n} else{(WORD)@a >>(-n)}
pcmpn	(B)a, (B)b, (B)lng	Compare two string	for(i=0;i<lng;i++)(B)@a[i]-(B)@b[i]	zflag, cflag
paddn	(B)a, (B)b, (B)lng	Add two string	for(i=0;i<lng;i++)(B)@a[i]+=(B)@b[i]

(B) or (BYTE) - 8-bit argument or operation
(W) or (WORD) - 16-bit argument or operation
a, b - address of data in processor memory (DATA/IDATA)
@a - data in processor memory at address a
@a[i] - data in processor memory at address a + i
imm - immediate constant
n - number of bits
lng - string length

The table of basic arithmetic operations unveils limited addressing options of individual operands - mostly immediate values are used. That is not entirely true; remember the INDirect flags of P-Code itself. For example, command
DPBB_ pmovwi, IND2, data_addr, wordtemp, blank
performs (WORD)@a = (WORD)@b, that is, assignment (WORD)data_addr = (WORD)wordtemp.
When using INDirect flags on BYTE commands, remember that INDirect expands as WORD and a BYTE command uses LSB of this expansion. So, it is necessary to remember that the operand is actually taken from @(operand+1), or to use INDirect parameters with SWAP flag set. Considering the power of INDirect flag, a closer look at the instruction set reveals that for example MOVB instruction is unnecessary.

Contents of LIBW, module PCODE

*instruction*	*parameters*	*description*	*operation*	*flags*
pcall	(W)addr	Call to P-Code subroutine	push current P-Code PC to stack, jump to addr
pret		Return from P-Code subroutine	pop P-Code PC from stack
pjump	(W)addr	Jump to P-Code addr	jmp addr
pjumpeq	(W)addr	Jump if zflag to P-Code addr	if(zflag) jmp addr
pjumpne	(W)addr	Jump if not zflag to P-Code addr	if(zflag == 0) jmp addr
pjumpCarry	(W)addr	Jump if cflag to P-Code addr	if(cflag) jmp addr
pjumpnCarry	(W)addr	Jump if not cflag to P-Code addr	if(cflag == 0) jmp addr

addr - address

The address specified as a parameter of a P-Code instruction must also point into a P-Code section. It is not possible to jump to a machine code instruction directly. Instead, P-Code section needs to be terminated with DB 0, and then an appropriate machine instruction such as jmp, jb, jnb, etc. is used. However, it is possible to call a machine code procedure from a P-Code section in a way similar to calling a P-Procedure - for example, with the macro DP machine_code, parameter1, parameter2, parameter3.
P-Code Interpreter, part 3