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DEC PDP-11 BASIC

Try the DEC BASIC Demo.

PCjs has archived the following DEC resources:

• BASIC (Single User) Tape Image
• PDP-11 BASIC PROGRAMMING MANUAL (December 1970)

Third-party resources include:

• "PDP-11 Paper Tape BASIC", written March 24, 2013

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Debugging Notes

One of the first things I noticed when debugging PDP-11 BASIC was its heavy reliance on TRAP instructions. For example, TRAP 000 is used to output the character in R2 to the terminal. Let's take a closer look at how its TRAP handler works.

First, if you check the table of PDP-11 trap vectors, you'll see that the vector for TRAP instructions is 000034. So let's dump the contents of the two-word vector at 000034:

>> dw 034 l2
000034  000100  000000  

The BASIC TRAP handler is at 000100, and here's that code:

>> u 000100
000100: 011666 000002          MOV   @SP,2(SP)
000104: 162716 000002          SUB   #2,@SP
000110: 013646                 MOV   @(SP)+,-(SP)
000112: 006216                 ASR   @SP
000114: 103404                 BCS   000126
000116: 006316                 ASL   @SP
000120: 062716 073654          ADD   #73654,@SP
000124: 013607                 MOV   @(SP)+,PC

When the code starts, the TRAP instruction has already pushed two words onto the stack:

0(SP):  previous PC
2(SP):  previous PSW

The first instruction, MOV @SP,2(SP), copies the previous PC onto the previous PSW, which is where we'll eventually want previous PC, so that the handler can eventually return with a simple RTS PC.

The next instruction, SUB #2,@SP, subtracts 2 from the original previous PC, so that it now points to the TRAP instruction.

Then @(SP)+,-(SP) fetches the TRAP instruction while also "popping" the original previous PC and then "pushing" TRAP instruction onto the stack, overwriting the original previous PC.

The next few instructions shift the TRAP right to see if bit 0 is set, and if it is not, then the TRAP instruction is restored by shifting it left again, and then a large offset is added to it, transforming the TRAP instruction (which is now known to be an even value) into a jump table index.

The final instruction, MOV @(SP)+,PC, moves the address at the jump table index into PC, while also removing the TRAP instruction from the stack, leaving only the previous PC on the stack.