Skip to content

Latest commit

 

History

History
122 lines (103 loc) · 3.63 KB

palx.md

File metadata and controls

122 lines (103 loc) · 3.63 KB
Raw

PALX assembler for the PDP-8/X

Like most assemblers, PALX is line oriented. However, a semicolon is equivalent to a line break, except inside a comment.

Line layouts

[label,] expr* [/comment]
[label,] nonmri [/comment]
[label,] mri [I] [Z] addr [/comment]
[label,] pseudo-op [/comment]

Expression primitives

name - value of name
number without decimal point - hex integer
number with trailing decimal point - decimal integer
number with non-trailing decimal or exponent - 32-bit IEEE float
"..." - 1 to 4 character constant (packed little-endian)
(expr) - address of newly allocated word on this page whose value is expr
[expr] - address of newly allocated word on pagef zero whose value is expr
. - current address

Expression operators

+ - integer addition
- integer subtraction/negation
& - bitwwise and
| - bitwise or
# - low half on the left, high half on the right

Mixing absolute and relocatable values

Constants and non-label names are absolute, labels are relocatable.

absolute <op> absolute = absolute
relocatable + absolute = relocatable
relocatable - absolute = relocatable
relocatable - relocatable = absolute

Everything else is an error.

Pseudo-ops

*addr - set assembly address (absolute or relocatable)
name = expr - define name
asect [name] - start assembling a new absolute section
base addr - set the assembler's idea of the FPP base page
block n [k] - assemble a block of n words filled with k (default 0)
common [name] - start assembling a new common section
commz [name] - start assembling a new common section in page zero
cpage n - ensure the next n words are on the current page
eap - enter auto-paging (CPU code can cross pages)
extern name - name is external (may be defined or undefined)
index addr - set the assembler's idea of the FPP index register block
lap - leave auto-paging (CPU code cannot cross pages)
page [n] - start assembling at the beginning of page n, or next page
reloc addr - assemble in current location as if at addr
sect [name] - start assembling a new relocatable section with CPU code
skipdef name=addr - define new skip instruction
text /foo/ - assemble UTF-8 into words with NUL termination (little-endian)
word - assemble 0x0000 if high half, nothing if low half
       (implied after jump instructions or before datums or labeled instructions)

Notes

INC is equivalent to ISZ, but the programmer guarantees it won't skip.

MRI instructions:

  • AND, TAD, ISZ, DCA, JMS, JMP
  • MUY, DVI, NMI, DAD. DST (do not use Z)
  • FLDA, FADD, FSUB, FDIV, FMUL, FADDM, FSTA, FMULM
  • JEQ, JGE, JLE, JA, JNE, JLT, JGT, JAL
  • SETX, SETB, JSA, JSR, JXN
  • TRAP3, TRAP4, TRAP5, TRAP6, TRAP7

Page zero subroutine for off-page indirection in EAP mode

          *07FA
saveac,   0
indirect, 0
offpage,  0
          dca saveac     / save AC
          tad i offpage  / fetch word after call to offpage
          isz offpage    / on return, skip word just fetched
          dca indirect   / stash the address to indirect through
          tad saveac     / restore AC
          jmp i offpage  / return from subroutine; last instruction on page zero

The calling sequence is:

          jms i offpage
          address
          <mri> i indirect

where is the operation we want to do.

Instruction sequences at end of code area in EAP mode

          / last instruction, non-skip
          jmp last
          / assembler-inserted pointers and values
last,     nop            / last instruction on page

          / last instruction, skip
          jmp last-1
          jmp last
          / assembler-inserted pointers and values
          skp
last,     skp            / last instruction on page