diff --git a/src/common/parser/EscapeSequenceParser.ts b/src/common/parser/EscapeSequenceParser.ts
index c24e5950..364ba803 100644
--- a/src/common/parser/EscapeSequenceParser.ts
+++ b/src/common/parser/EscapeSequenceParser.ts
@@ -216,13 +216,22 @@ class DcsDummy implements IDcsHandler {
  * EscapeSequenceParser.
  * This class implements the ANSI/DEC compatible parser described by
  * Paul Williams (https://vt100.net/emu/dec_ansi_parser).
+ *
  * To implement custom ANSI compliant escape sequences it is not needed to
  * alter this parser, instead consider registering a custom handler.
  * For non ANSI compliant sequences change the transition table with
  * the optional `transitions` contructor argument and
  * reimplement the `parse` method.
- * NOTE: Other than the original parser from vt100.net this parser supports
- * sub parameters in digital parameters separated by colons.
+ *
+ * This parser is currently hardcoded to operate in ZDM (Zero Default Mode)
+ * as suggested by the original parser, thus empty parameters are set to 0.
+ * This this is not in line with the latest ECMA specification
+ * (ZDM was part of the early specs and got completely removed later on).
+ *
+ * Other than the original parser from vt100.net this parser supports
+ * sub parameters in digital parameters separated by colons. Empty sub parameters
+ * are set to -1.
+ *
  * TODO: implement error recovery hook via error handler return values
  */
 export class EscapeSequenceParser extends Disposable implements IEscapeSequenceParser {
@@ -261,7 +270,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
     this.currentState = this.initialState;
     this._osc = '';
     this._params = new Params(); // defaults to 32 storable params/subparams
-    this._params.addParam(0);
+    this._params.addParam(0);    // ZDM
     this._collect = '';
     this.precedingCodepoint = 0;
 
@@ -392,7 +401,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
     this.currentState = this.initialState;
     this._osc = '';
     this._params.reset();
-    this._params.addParam(0);
+    this._params.addParam(0); // ZDM
     this._collect = '';
     this._activeDcsHandler = null;
     this.precedingCodepoint = 0;
@@ -502,7 +511,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
           do {
             switch (code) {
               case 0x3b:
-                params.addParam(0);
+                params.addParam(0);  // ZDM
                 isSub = false;
                 break;
               case 0x3a:
@@ -528,7 +537,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
         case ParserAction.CLEAR:
           osc = '';
           params.reset();
-          params.addParam(0);
+          params.addParam(0); // ZDM
           collect = '';
           break;
         case ParserAction.DCS_HOOK:
@@ -558,7 +567,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
           if (code === 0x1b) transition |= ParserState.ESCAPE;
           osc = '';
           params.reset();
-          params.addParam(0);
+          params.addParam(0); // ZDM
           collect = '';
           break;
         case ParserAction.OSC_START:
@@ -605,7 +614,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
           if (code === 0x1b) transition |= ParserState.ESCAPE;
           osc = '';
           params.reset();
-          params.addParam(0);
+          params.addParam(0); // ZDM
           collect = '';
           break;
       }
diff --git a/src/common/parser/Params.ts b/src/common/parser/Params.ts
index 8fd86afc..7ef8341a 100644
--- a/src/common/parser/Params.ts
+++ b/src/common/parser/Params.ts
@@ -22,6 +22,11 @@ const MAX_SUBPARAMS = 256;
  *    - max. value for a single (sub) param is 2^31 - 1 (greater values are clamped to that)
  *    - max. 256 sub params possible
  *    - negative values are not allowed beside -1 (placeholder for default value)
+ *
+ * About ZDM (Zero Default Mode):
+ * ZDM is not orchestrated by this class. If the parser is in ZDM,
+ * it should add 0 for empty params, otherwise -1. This does not apply
+ * to subparams, empty subparams should always be added with -1.
  */
 export class Params implements IParams {
   // params store and length
diff --git a/src/common/parser/Types.d.ts b/src/common/parser/Types.d.ts
index 5f925690..d432800d 100644
--- a/src/common/parser/Types.d.ts
+++ b/src/common/parser/Types.d.ts
@@ -6,18 +6,18 @@
 import { IDisposable } from 'common/Types';
 import { ParserState } from 'common/parser/Constants';
 
-/**
- * Params types.
- */
+/** sequence params serialized to js arrays */
+export type ParamsArray = (number | number[])[];
+
+/** Params constructor type. */
 export interface IParamsConstructor {
   new(maxLength: number, maxSubParamsLength: number): IParams;
 
-  /** create params object from array like [1, [2, 3]] */
-  fromArray(values: (number | number[])[]): IParams;
+  /** create params from ParamsArray */
+  fromArray(values: ParamsArray): IParams;
 }
 
-export type ParamsArray = (number | number[])[];
-
+/** Interface of Params storage class. */
 export interface IParams {
   /** from ctor */
   maxLength: number;
@@ -100,13 +100,14 @@ export interface IEscapeSequenceParser extends IDisposable {
    * It gets reset by the parser for any valid sequence beside REP itself.
    */
   precedingCodepoint: number;
+
   /**
    * Reset the parser to its initial state (handlers are kept).
    */
   reset(): void;
 
   /**
-   * Parse string `data`.
+   * Parse UTF32 codepoints in `data` up to `length`.
    * @param data The data to parse.
    */
   parse(data: Uint32Array, length: number): void;